Answer

Jain Chaturmas / Choumas traditionally ends on Kartik Sud Poonam — the full moon day of the month of Kartik.

This day is also known as Dev Diwali in many Jain communities.

Explanation

Jain Choumas generally begins from Ashadh Sud Chaudas / Poonam and continues for about four months. During this period, Jain monks and nuns stay in one place and do not travel, following the discipline connected with the rainy season.

It ends on Kartik Sud Poonam, after which monks and nuns may resume vihār according to their tradition and circumstances.

Takeaway

So, in simple terms: Jain Choumas ends on Kartik Sud Poonam.