Answer

Chomasu, or Chaturmas, usually begins in the month of Ashadha. Jain references do show slightly different starting tithis: some mark the beginning on Ashadha Chaturdashi, while many modern Jain calendars note it from Ashadha Shukla Ekadashi. ( jainworld.com)

Explanation

So, the exact starting day can vary by tradition and local Jain sangh, because Chaturmas is observed according to the lunar calendar. It is the rainy-season period when Jain sadhus and sadhvis stay in one place for spiritual discipline. ( jainworld.jainworld.com)

Takeaway

If you mean the general Jain Chomasu period, think of it as starting in Ashadha, with the exact tithi depending on the tradition followed locally. ( jainworld.com)