Mahā Śivarātri — the niśītha rule
The popular claimThe great night of Śiva is fixed to the middle of the night — the one time of day no calendar prints.
Śivarātri is a night. That sounds obvious, and it is the reason the date is so often printed a day out: calendars have a column for dates and none for the hour after midnight.
The rule
The tithi is Phālguna kṛṣṇa Caturdaśī — the fourteenth of the dark half, the night before the new moon of Phālguna. In the pūrṇimānta reckoning used in the north that is the month of Phālguna; in the amānta reckoning of the south the same night falls in Māgha, which is why southern calendars call it Māgha Śivarātri and mean the same night.
The observance is taken on the day whose niśītha falls within Caturdaśī. Niśītha is the middle muhūrta of the night: the night from sunset to sunrise is divided into fifteen muhūrtas, and the eighth is niśītha — roughly forty-eight minutes centred a little after midnight. If Caturdaśī covers niśītha on one night, that is Śivarātri. If it covers it on both of two nights, the nibandhas take the second — the one adjoining amāvasyā — and if neither, the night on which Caturdaśī is present in the greater part of the pradoṣa-to-niśītha stretch.
2027, computed
Caturdaśī begins at 12:04 PM on Saturday 6 March and ends at 1:46 PM on Sunday 7 March — Delhi, this site's engine.
Saturday's night: sunset 6:22 PM, sunrise 6:41 AM the next morning, a night of twelve hours nineteen minutes. Each muhūrta is 49 minutes; the eighth runs from about 12:07 AM to 12:56 AM on the 7th. Caturdaśī covers it. Sunday's night: Caturdaśī ended at 1:46 in the afternoon, so its niśītha is amāvasyā. Only one night qualifies. Mahā Śivarātri 2027: the night of Saturday 6 March.
The four prahara pūjās divide that same night into quarters of about three hours and five minutes each: the first from sunset, 6:22 PM to 9:27 PM; the second to 12:31 AM; the third to 3:36 AM; the fourth to sunrise at 6:41. The niśītha falls in the second prahara, which is why the second is the one the texts weight most.
What the night is for
The Śiva Purāṇa, in the Vidyeśvara Saṁhitā, gives the vrata: a fast through the day, a vigil through the night, worship of the liṅga in each prahara. It also gives the night its story — the appearance of the liṅga of light before Brahmā and Viṣṇu, neither of whom could find its end. Later tradition adds the marriage of Śiva and Pārvatī, and some regions keep the night for that.
What is constant across the sources is the shape: it is a vigil, and the whole discipline is to stay awake through the muhūrta the day is named for.
The fast that accompanies it ends with the pāraṇa on the following day, and the nibandhas place the pāraṇa after Caturdaśī has ended — in 2027, after 1:46 PM on Sunday 7 March — or, in some traditions, at sunrise on the 7th if the tithi will end before the day's third part. Either way, it is not broken at midnight; the vigil and the fast are one observance and both run to the morning.
Unlike Navrātri or Diwāli, Śivarātri has almost no regional variation in its date, because the niśītha rule so rarely produces two candidate nights. What varies is the number of Śivarātris kept: the fourteenth of every dark fortnight is a māsa-śivarātri, kept by some, and the Phālguna one is the great one — mahā — among twelve.
Why the times above are not the times for you
Niśītha is computed from your own sunset and the next sunrise. In March, Toronto's night is about an hour longer than Delhi's and starts at a different clock hour; the tithi boundary converts by the time difference. In 2027 both cities land on the same night, but that is a result, not a rule. This site's panchang computes the niśītha for the city you type.
What this page does not say
It does not say what the vigil earns, and it does not warn about the year someone sleeps through it. The Purāṇa describes a night and the nibandha fixes which night; that is what is computed and shown. The rest is between you and the night.
Source — Dharmasindhu and Nirnayasindhu on nishitha-vyapini chaturdashi for Shivaratri and on the four-prahara worship; the Shiva Purana (Vidyeshvara Samhita) and Linga Purana on the vrata; sunset, sunrise and tithi boundaries computed by this site's panchang engine
Every claim on this site names its source, and every calculator shows its arithmetic.