Navroz is the Parsi new year's day. The day before is Pateti, the last day of the previous year, and the day to close accounts for the year.
The significance of Pateti is that it is the day to dwell on the wrongs or sins one may have committed the previous year, and atoning for them.
While Pateti is generally observed in August, it is not universal among the Parsis. The Parsis follow one of three calendars. It is the Shahenshahi, and most common, calendar, which has Pateti and the new year in August. The other calendars are the Fasli (the old Persian calendar), which has the day in March, and the Kadimi, which celebrates the festival one month before the Shahenshahis.
The word pateti is derived from Pazend patet, meaning 'repentance'. The Zoroastrian tenets are based on the three ideals of good thoughts, good words and good deeds. Anything that is out of accord with this is considered a sin. It is natural for anyone to have committed a sin in the course of a year, even if only inadvertently. Pateti is the occasion to redeem oneself by offering patet, or the prayer of repentance, and prepare to greet the new year with a clean conscience.
It is also a day of thanksgiving, to be grateful to God not just for the joys of life but also the sorrows.
On the day of Pateti, the Parsis dress up in new clothes, visit the fire temples, offer charity and arrange lavish feasts in their homes. Meals consist of traditional Parsi dishes, including pulao dal, sali boti, and patra-ni-machchi.
A few years ago, Gujarati theatre and comedies were also part of the Pateti celebrations. After a meal and wishing friends and families, the Parsis would get together to watch the nataks, humorous plays usually full of double entendres.
Most of the Parsis in India are followers of the Shahenshahi calendar. So, in India, Pateti falls in August. Outside India, most Zoroastrians follow the Fasli calendar. So, the festival falls in March.