Two days a month, at the ends of the two fortnights, and both carry observances.
They are also the point at which yoga culture makes its most confident unsupported claims, so this article does two things: sets out what the tradition actually does on these days, and states plainly what the evidence says about the moon and the body. Those are separate questions and conflating them is the usual error.
The Two Days
Pūrṇimā, the full moon. The fifteenth tithi of the bright fortnight, where the waxing fortnight ends.
Amāvasyā, the new moon. The fifteenth tithi of the dark fortnight, where the waning fortnight ends. The word relates to dwelling together, in the sense of the sun and moon occupying the same position.
Each occurs once per lunar month, roughly a fortnight apart, as the calendar article sets out. Whether a given civil date is the full or new moon day depends on the tithi convention, so a pañcāṅga is the reference.
What the Tradition Does
The observances are distinct and the difference is consistent across traditions.
Pūrṇimā is the auspicious one. Several of the major full moons are festivals in their own right: Guru Pūrṇimā in Āṣāḍha, Holi at the Phālguna full moon, Buddha Pūrṇimā in Vaiśākha, Kārtika Pūrṇimā with its lamp observances. Full moons are days for bathing, for giving, for fasting in some traditions, and for beginning things.
Amāvasyā is treated differently. It is the day associated with the ancestors, and tarpaṇa and śrāddha rites, offerings to the departed, are performed on it. Mahālaya Amāvasyā, before the autumn Navarātri, is the principal ancestral observance of the year. Some new moons are inauspicious for beginning things.
And Diwali is the exception that confirms the pattern. It falls on an amāvasyā, the darkest night of Kārtika, and the observance is to fill it with lamps. The festival works because the night is dark.
Two things to take from this. The full moon is for the living and the new moon is for the dead, broadly, across a great deal of the ritual calendar. And the fortnightly alternation gives the month a shape: a waxing half building to a bright day, a waning half ending in a dark one.
The Major Full Moons
Worth listing, since several are festivals and a student will meet them by name.
Vaiśākha Pūrṇimā, April or May. Buddha Pūrṇimā, marking the Buddha's birth, awakening and death on one day in the Theravāda reckoning.
Āṣāḍha Pūrṇimā, June or July. Guru Pūrṇimā, and in the Buddhist calendar also the first sermon.
Śrāvaṇa Pūrṇimā, July or August. Rakṣā Bandhan, the tying of a thread between siblings, and in parts of the south the day on which the sacred thread is renewed.
Āśvina Pūrṇimā, September or October. Śarad Pūrṇimā, the harvest full moon, held to be the brightest of the year.
Kārtika Pūrṇimā, November or December. Lamp observances on the river, and a major bathing day.
Phālguna Pūrṇimā, February or March. Holi.
Two observations. The full moons carry a disproportionate share of the festival calendar, which is consistent with their auspicious standing. And several are shared with Buddhism, where the full moon days, uposatha, are the principal observance days, so a student in a mixed setting may find the same date observed for different reasons.
The Moon Days in Ashtanga
Worth addressing specifically, since this is where most yoga practitioners encounter the idea.
In the Ashtanga tradition, full and new moon days are rest days: practice is suspended. The stated reasoning, as it is usually given, involves the moon's effect on the body's fluids and on energy, with the full moon described as expansive and the new moon as contracted.
Three honest observations.
It is a real and consistent institution. Whatever one makes of the reasoning, the practice of taking two rest days a month at fixed points is well established in that tradition and has been for decades.
The physiological reasoning is not supported. There is no good evidence that lunar phase affects the body's fluid balance, injury risk, or capacity for physical practice.
And the institution does not need the reasoning. Two scheduled rest days a month, at predictable intervals, is good practice for a demanding daily physical discipline. A practitioner who takes them because the schedule says so, without the physiology, gets the entire benefit.
That is the most useful thing to say about moon days: keep them, and drop the explanation.
What the Evidence Actually Says
This needs stating carefully, because claims in both directions are overstated.
On human behaviour and emergency presentations. The folklore that full moons increase psychiatric admissions, emergency room attendance, crime or births has been studied extensively and the reviews do not support it. This is one of the better-studied popular beliefs and the finding is consistently negative.
On sleep. There is some published work suggesting small effects of lunar phase on sleep duration and timing, and the literature is mixed with failures to replicate. It is not a settled finding and the reported effects, where found, are small.
On the body's fluids. The tidal analogy, that the moon moves the body's water as it moves the sea, does not work. Tides are a gradient effect across the diameter of a large body, and the gradient across a human being is negligible.
And on menstrual cycles. The approximate coincidence of cycle length with the lunar month is a coincidence of duration and is not synchronisation; the research does not support cycles tracking lunar phase.
What follows is narrow. A practitioner who finds they sleep or practise differently around the full moon is reporting something about themselves, which is worth attending to as information about them. It is not evidence of a general effect, and the general claims should not be made in a class.
Practising With the Fortnights
If the physiology is out, the question is whether there is anything left. There is, and it is structural.
A fortnightly rhythm is a good interval for a practice cycle. Long enough to build something, short enough to be reviewed. Two weeks is the period over which a practitioner can reasonably assess whether something is working.
Marked days are useful whatever marks them. A fixed point for rest, or for a longer sitting, or for review, works because it is fixed and not because of what it coincides with. The moon supplies an unambiguous, universally visible marker that requires no calendar.
And the full moon is genuinely visible. A practice anchored to something you can walk outside and look at is easier to keep than one anchored to a date.
So: use the days, and be accurate about why. The moon is a reliable fortnightly signal, free, and visible from anywhere, which is a sufficient reason to organise a rhythm around it.
What a Practitioner Can Use
Two rest days a month, fixed. The Ashtanga institution without the physiology. For anyone practising daily, scheduled rest at predictable intervals is sound.
The full moon for beginning, the new moon for finishing. The tradition's own broad pattern, and a serviceable frame for a practice cycle regardless of belief.
The ancestral observance on amāvasyā. Whatever one holds about the dead, a day each month designated for remembering them is a humane institution and it costs nothing to keep.
And honesty about one's own responses. If you practise differently at the full moon, that is worth noticing about yourself and is not a fact about the moon.
For Teachers
Separate the two questions. What the tradition does on these days, and what the moon does to a body. Both are legitimate subjects and conflating them is the usual error.
Teach the moon days as a schedule. Two fixed rest days a month is good practice for a daily discipline. That is defensible without any claim about fluids.
Do not make the physiological claims in a class. The behavioural research is consistently negative and the tidal analogy fails on the arithmetic. A teacher who asserts them is making a factual claim that is wrong.
Allow the individual report. A student who notices they sleep or practise differently at the full moon is telling you something about themselves. That is worth hearing and is not evidence of a general effect.
And give the ancestral observance its weight. Amāvasyā as the day for the dead is half of this pairing and it is almost never mentioned in yoga settings, which leaves students with the pretty half of a structure.
Frequently Asked Questions
What are Purnima and Amavasya?
Pūrṇimā is the full moon, the fifteenth tithi of the bright fortnight. Amāvasyā is the new moon, the fifteenth tithi of the dark fortnight. Each occurs once per lunar month.
What does the tradition do on the full moon?
Treats it as auspicious. Several major festivals fall on full moons, including Guru Pūrṇimā, Holi and Buddha Pūrṇimā, and the day is used for bathing, giving and beginning things.
What about the new moon?
It is associated with the ancestors, with tarpaṇa and śrāddha offerings performed on it. Mahālaya Amāvasyā is the principal ancestral observance of the year.
Why is Diwali on a new moon then?
Because it is a festival of lamps placed at the darkest night of the month. The observance works precisely because the night is dark.
Why does Ashtanga rest on moon days?
The institution is well established and the stated reasoning involves lunar effects on the body's fluids and energy. The institution is sound as a schedule of two fixed rest days a month; the physiological reasoning is not supported.
Does the moon affect human behaviour?
The folklore about psychiatric admissions, emergency attendances, crime and births has been studied extensively and the reviews do not support it. The finding is consistently negative.
Does it affect sleep?
Some published work suggests small effects on sleep duration and timing, and the literature is mixed with failures to replicate. It is not settled and the reported effects are small.
Does the moon move the body's fluids like tides?
No. Tides are a gradient effect across the diameter of a large body, and the gradient across a human being is negligible. The analogy does not work.
Are menstrual cycles synchronised with the moon?
The approximate coincidence of cycle length with the lunar month is a coincidence of duration. The research does not support cycles tracking lunar phase.
Which full moons are festivals?
Several: Buddha Pūrṇimā in Vaiśākha, Guru Pūrṇimā in Āṣāḍha, Rakṣā Bandhan in Śrāvaṇa, Śarad Pūrṇimā in Āśvina, Kārtika Pūrṇimā, and Holi at the Phālguna full moon. Several are shared with Buddhism, where full moon days are the principal observance days.
So is there any reason to practise with the moon?
A structural one. A fortnightly rhythm is a good review interval, fixed marked days work because they are fixed, and the moon is a free, unambiguous and universally visible signal requiring no calendar.
Keep the Days, Drop the Physiology
The full moon is the tradition's auspicious day and the new moon its day for the dead, which gives the month a shape worth using. The Ashtanga institution of two monthly rest days is good practice for a demanding daily discipline and needs no account of fluids to justify it. The behavioural claims have been tested and have not held up, the tidal analogy does not survive the arithmetic, and none of that is a reason to stop marking the days. It is a reason to say accurately why you are marking them.
Rohit Yoga Academy runs a residential 200-hour Yoga Teacher Training in Swargashram, Rishikesh, where the full moon over the river is worth the walk and is not described as doing anything to you.