In Jain Dharma, Chaturmas (Chaumasa) does not have a fixed English (Gregorian) date every year.

By definition:

• Start of Chaturmas:

From Āṣāḍh Śukla Ekādashi (Devshayan Ekadashi) – 11th tithi of the bright half of the month of Āṣāḍh.

• End of Chaturmas:

Till Kārtik Śukla Ekādashi (Devuthan / Prabodhini Ekadashi) – 11th tithi of the bright half of the month of Kārtik.

So Chaturmas is the four-month period from Āṣāḍh Śukla Ekādashi to Kārtik Śukla Ekādashi as per the Jain lunar calendar. Every year, these tithis fall on different English dates, and can also vary slightly by country/city, so Jains follow their local Jain Panchāng or Sangh calendar.

You can see a simple explanation here