Here is the basic meaning of the 1st day of Paryushan, for both main Jain traditions.

• Shwetambar (8 days, ends with Samvatsari)

- 1st day is the beginning (ārambh) of Paryushan. On this day Jains make a sincere sankalp (resolve) to observe the fast and the path of purification for the coming days. Beginnings typically include starting pratikraman, studying scriptures (swādhyāya), observing samayik, and practicing tapas (austerities) with a firm vow to ahiṃsā (non-violence) and self-discipline. This sets the tone for the entire festival. You can read more here

• Digambar (Das Lakṣaṇa Parva, 10 days; ends with Anant Chaturdashi)

- 1st day is Uttam Kshama (supreme forgiveness). The day is dedicated to deep inner forgiveness—towards all beings and towards oneself—so that one can begin the festival with a pure heart. This sets the practice for the following days, which focus on other virtues (utsav of self-restraint, truth, non-possessiveness, etc.). For a brief outline of the Digambar approach, see the shared description here

If you’d like, I can point you to specific prayers or readings commonly done on Day 1 for your tradition.