The story
The night belongs to the churning of the ocean. When the poison rose before the nectar did, Śiva drank it and held it in his throat, and the gods waited out the evening not knowing whether he would live. He did, and danced between the horns of Nandi; the hour they waited is the hour kept.
That is why the vrata is an hour and not a day. It marks a particular evening rather than a particular date, and the almanac gives it to the minute because the hour is the whole of it.
Each weekday it falls on carries its own name and its own asking — Soma Pradoṣa on a Monday for a quiet mind, Śani Pradoṣa on a Saturday for relief from Śani himself.