A tithi is a lunar day — the time the moon takes to move 12 degrees relative to the sun. Each lunar month has 30 tithis in all, split across the Shukla and Krishna paksha. Since the moon's speed is not uniform, a tithi may be shorter or longer than one English day, and two tithis can even fall on the same day.
For this reason the panjika lists the specific start and end time of each tithi. Knowing these times is essential for fixing the correct day for vratas, fasting and puja — for example, the Ekadashi fast is observed within the tithi's specific window. The MandirBD panjika shows the exact tithi timing, nakshatra and auspicious or inauspicious yoga each day.