In Jain Dharma, Chaturmas (Chaumasa) generally means:

• Starts: Ashadh Shukla Ekadashi (Devshayani Ekadashi)
• Ends: Kartik Shukla Ekadashi (Devutthani/Prabodhini Ekadashi)

So, in simple words:

> Chaturmas is from Ashadh Shukla 11 to Kartik Shukla 11 (according to the lunar calendar).

Every year the English (Gregorian) dates change, so for a specific year you should check a Jain Panchang or your local sangh’s calendar.

You can read a short Jain-focused explanation here: