How the date is fixed
The Krishna Chaturdashi of every lunar month. The date is the day on which that tithi is running at the middle of the night (halfway from sunset to the next sunrise). Dark-half entries carry both month names: purnimanta (North India) and amanta.
About
Masik Shivaratri is the fourteenth tithi of the dark half of every lunar month, the night before the new moon. Like Maha Shivaratri it is a night observance, so the day whose middle of the night (nishita) carries Chaturdashi is chosen, even when the tithi is absent at sunrise. The one in amanta Magha (Phalguna in the north) is Maha Shivaratri itself, and the one in Shravana as North India counts it is often called Sawan Shivaratri. Each entry shows its month in both reckonings, since North India names the dark half one month ahead.
How it is observed
A fast through the day and worship of Shiva at night, with abhisheka of the linga.