Shiva's Post

The Shiva fast (Pradosh vrat) is observed by adherents of Shaivism on the 13th lunar day on the waning and growing Moon (twice a month).
That day, at the time of sunset of which the 13th lunar day falls, is considered a day of fasting.
And, since in this case there is a binding to the sunset times, it is important to understand that these may be different days for different cities.