“Trishalā Mātā na Sapna” are the auspicious dreams seen on the night of Bhagwān Mahāvīr’s Garbha (Chyavana) Kalyanak. Their traditional tithi is:

• Āshādh Shukla 6 (the 6th day of the bright half of Āshādh).

Traditions:

• Śvetāmbara: 14 dreams are described.
• Digambara: 16 dreams are described.

How it’s observed today:

• Temples especially recall and display these swapna on Mahavir Janma Vanchan during Paryushan (Śvetāmbara) and also around Mahavir Janma Kalyanak (Mahavir Jayanti). The exact Gregorian date varies each year with the lunar calendar.

For a simple overview of Mahāvīr’s Kalyanaks and the swapna, see this concise note from JainKnowledge: and a general Kalyanak-dates explainer: