Why Kashi made almanacs
A temple town needs a calendar: the day of every festival, the hour of the abhisheka and the time of the Ganga aarti had to be known beforehand. In Kashi, where scholars of Jyotisha taught and worked at the ghats and in the schools of Sanskrit, the calculation of the panchang grew into a craft, and the almanacs of the city came to be followed in places far from the Ganga. The Vishwa Panchang of Banaras Hindu University is the scholarly work of the same tradition.
What the Kashi panchangs follow
Most of the Varanasi almanacs follow the Purnimanta month, in which the dark half of a month falls before its full moon, so that a festival of the dark half is placed in the month that ends on the coming full moon. The year is counted in the Vikram Samvat and the Saka. The times of a day are worked out for Kashi, and many printed tables of sunrise and sunset in the book are for the city, with a correction given for other places.
Alongside the five limbs of the day, the panchang gives the lists of the festivals of the year, the days of the fasts of each month, the muhurtas for samskaras and for new beginnings, and the Sankranti and the eclipses. Some editions add the Ganga snan days and the pilgrimage dates that matter in Kashi.
Which panchang to follow
A family settles the day of a ceremony by the almanac of its own tradition, and the priest who performs it will usually follow the panchang he has used for years. This site calculates the same limbs from the positions of the Sun and Moon and shows the times for any place, so that a reader can see when a tithi begins and ends and why two almanacs may call different days for the same observance.