Short answer: Within the Śvetāmbara community, “different tithi” (festival/vrat dates differing by a day) happens mainly because of differences in how the lunar day is calculated and applied locally.

Key reasons, in simple words:

• Different Panchāng (almanacs): Various Śvetāmbara groups use different Jain panchāng/nirṇay (calculation rules). Small differences in calculation lead to different tithis.
• Sunrise rule vs other rules: Many Jain vrats/festivals are kept by the tithi present at local sunrise (vyavahār tithi). Some follow slightly different cut‑off rules (e.g., whether a tithi must “touch” sunrise, what to do in kṣaya/vr̥ddhi tithi), so the observed day can shift.
• Place and time zone: Tithi is based on the Moon’s position at your location. India, UK, USA, etc., can fall on different civil dates for the same lunar tithi.
• Kṣaya/vr̥ddhi tithi: Sometimes a tithi is very short (skips a sunrise) or very long (covers two sunrises). Different traditions handle these special cases differently.
• Gaccha/saṅgh practice: Within Śvetāmbara (e.g., Mūrtipūjak, Sthānakvāsi, Terāpanth, and regional saṅghs), long‑standing local decisions are followed to keep uniformity in each community.

What to do:

• Follow the Panchāng and announcements of your own local saṅgh/derāsar. That preserves unity and the intent of the vrata.
• If you travel or live abroad, check the local Jain center’s schedule because the tithi may shift with the time zone.

For simple date explanations (why US/India dates differ, how annual dates are set), you can refer here: and here: