The night when Shiva is most accessible — vigil and fast through the four prahars.
Maha Shivaratri is the most sacred Shiva night of the year, falling on Phalguna Krishna Chaturdashi. Devotees keep an all-night vigil with fasting, abhishekam of the Shivalingam, and recitation of Shiva mantras through the four prahars (3-hour watches) of the night.
Observed when Chaturdashi tithi is present at Nishita Kala (the midnight watch). When Chaturdashi straddles two civil days at sunrise, the night where Chaturdashi covers nishita is the observance night — typically the day before the sunrise-Chaturdashi day.
Computed for San Cosme y Damian (America/Asuncion) using local sunrise, sunset and astronomical positions.