In the linga purana, , there is a story of a hunter (nishada) who, when bent on killing a doe, her mate and children unknowingly performed all the characteristic actions of Sivaratrivrata and the family of the buck ultimately became transformed into the Mrgasirsa nakshatra with the star of the hunter (sirius) behind.
There is a conflict of views about what the chief thing in Sivaratri is. The Tithitatva holds that a fast is the principle thing in sivaratri and relies on the words of sankara: "On that tithi(sivaratri), I am not so pleased with bath (on the linga), nor with clothes again (offered to the Linga) nor with inscence, nor by worship, nor with the offering of flowers, as I am pleased by a fast". On the other hand, hemadri and madhava hold that Sivaratri means fast, worship and jagarana and all the three are apprehended as principle and quote passages from the Skanda purana and Nagara khanda in support. "The man who observes a fast on Sivaratri for 12 years who keeps awake (in dance and music) before me the whole night would reach heaven that man, who worships Siva and performs jagara on the 14th, would not there after taste milk at mother's breast(i.e would not be born again); the nishada, unknowingly worshipping a self existant Linga an observing a fast and jagara, became free from sin and attained the position of the attendant of Siva".
Sivaratri vrata is both nitya (obligatory) and kamya (optional). It is nitya, because there are passages which laydown that sin follows if a man does not perform it.... The vrata is optional, because the texts provide the rewards of performing these vrata.

The proper time for this vrata is night. Since, as started in the skanda, 'goblins, the saktis and Siva(who weilds the trident) move about at night and therefore these are to be worshipped at that time on chaturdasi'. The skanda puranas provides: "a fast should be observed on that chaturdasi of the dark half that is joined in nisitha(i.e.that exists at that time); tha tithi is the best and leads to companionship with Siva." Hemadri quotes nagarakhanda as follows: "on the 14th of the dark half of Magha, I shall go in the kaali yuga on the surface of the earth at night and not by day. I shall undoubtedly transfer myself to all Lingas, whether movable or permanently fixed for the removal of the sins committed by man in the(proceeding) year; therefore the man who offers worship to me at night with these mantras will be free from sin." In the isanasamhita, it is said:"The first God (Siva) appeared in the form of Sivalinga, whose refulgence was equal to that of crore of Suns, on the 14th of Magha dark half. The tithi to be accpepted for the sivaratri vrata should be one that covers the time. A man should perform this on that tithi when the 14th covers the time before and after midnight." Hemadri also quotes a verse like this:"the 14th called Sivaratri that covers pradosha should be accepted(for vrata); one should observe a fast on that fast on that tithi since there has to be a jagara(in that vrata) at night.

From history of Dharmasastra Vol. V part I