In short
Paryushan is the holiest festival in Jainism, observed during the monsoon retreat. Shwetambar Jains observe eight days ending in Samvatsari; Digambar Jains observe ten days as Das Lakshan Parva, followed by Kshamavani. Both are days of fasting, scripture, reflection and forgiveness.
Paryushan is the most sacred observance in the Jain year. The word carries the sense of drawing near and abiding — coming close to the soul, and staying there long enough for something to change.
Why Paryushan falls in the monsoon
Paryushan sits inside Chaturmas, the four-month monsoon retreat during which Jain ascetics stop travelling. The rains bring an explosion of small life — insects, sprouting seeds, organisms in every puddle — and walking from place to place would destroy it constantly. Staying still is the ahimsa. Because the ascetics are settled and teaching in one place, the lay community has its most concentrated period of instruction of the year, and Paryushan is its summit.
The two observances
Both traditions keep the same festival, differently. Neither is a variant of the other; each is complete in itself.
Digambar — Das Lakshan Parva
Ten days, each given to one of the ten supreme virtues. Followed by Kshamavani, the day of forgiveness.
Shwetambar — Paryushan Parva
Eight days, with the Kalpa Sutra read aloud, culminating in Samvatsari, the day of forgiveness.
Both traditions are presented here with equal respect.
What devotees actually do
The observance rests on four things, in roughly equal measure. Fasting and dietary restraint, undertaken within one's own capacity. Scripture — heard daily, and studied. Pratikraman, the structured review of one's conduct. And forgiveness, sought and granted on the final day. The first three exist to make the fourth possible.
The history of the observance
The monsoon retreat is among the oldest practices in the tradition, recorded in the earliest layers of Jain literature as a rule of conduct for ascetics. The Kalpa Sutra, attributed to Acharya Bhadrabahu Swami, sets out the Samachari — the conduct required during the rains — alongside the lives of the Tirthankaras. Its public reading during Paryushan is what gives the Shwetambar observance its shape. The Digambar arrangement of ten days around the ten virtues draws on the same source material, organised by dharma rather than by narrative.
Who can take part
Anyone. Paryushan asks for participation according to capacity, not perfection. A person who cannot fast can still keep silence, avoid harsh speech, attend pravachan, give to those in need, and ask forgiveness sincerely on the final day. That is a complete observance.