There is no single universal majority for all Jains.

But in common Jain calendar usage, especially in western India and among many Shwetambar communities, the month is usually counted as _amavasya-ante_ (amavasyant), meaning the lunar month ends on new moon day.

A few notes:

• Amavasya is very important in Jain observance.
• Purnima is also very important, but it is usually not the month-ending basis in the most common Jain local calendar style.
• Regional practice can vary, so some communities may use different reckoning.

So, in short: most Jain everyday religious calendar usage is generally amavasya-ante, not purnima-ante.