Poṣh-daśamī (Posh‑daśamī) is a Jain festival that commemorates the birth of Lord Parshvanath. It is traditionally observed on the 9th, 10th, and 11th days of the dark half of the Poṣha (Pauṣa) month. The exact Gregorian dates change every year because the tithi (lunar day) calculations differ by region (north Indian vs Gujarati/Mārgaśīrṣa calendars) and by local lunisolar calendars. The Attham refers to a three-day fast that many Jains undertake during these days. ( jainknowledge.com)

What you can do to know the exact date for your year and region:

• Tell me the year and your region (e.g., North Indian calendar region, or Gujarati calendar region). I can give you the precise Gregorian dates for Poṣa‑daśamī and the Attham days for that year.
• If you’d like, I can fetch the exact dates for a specific year right away.

You can read more here and here