In our Jain lunar calendar:

• 1 tithi = Pratipada (પ્રતિપદા / प्रतिपदा)
• 2 tithi = Dwitiya / Biji (દ્વિતીયા / द्वितीया)

Both are lunar days, but they differ only in order and count:

1. Order / Numbering

- 1 tithi (Pratipada) = first day of the paksh (Shukla or Krishna) - 2 tithi (Dwitiya) = second day of the same paksh

1. Chandramā (moon) position

- Each tithi is defined by how far the moon has moved from the sun. - 1 tithi: moon is 0–12° away from the sun - 2 tithi: moon is 12–24° away from the sun (you don’t need to calculate this; the panchāng does it)

1. In Jain practice

- Both are normal days in general. - Some vrat, pūjā or parva may be fixed on a particular tithi (for example, some local traditions may give importance to a specific biji / bija tithi), but there is no general rule that 1 tithi is “better” and 2 tithi is “worse” or vice‑versa. - Spiritually, every day is equal for practicing samyak darshan, gyaan, chāritra; tithi is mainly for calendar and scheduling of religious activities.

So, difference is only: first lunar day vs second lunar day (Pratipada vs Dwitiya), based on the moon’s position and calendar counting.

If you want a simple explanation of “tithi kya hoti hai” in Jain panchāng style, you can read more here