“Sumati Jin diksha” on a Jain panchang usually refers to the Diksha Kalyanak of Bhagwan Sumatinath — that is, the auspicious day when Lord Sumatinath renounced worldly life and accepted ascetic life. “Jin” here means Jina/Tirthankar, and “Sumati” is a shortened form referring to Sumatinath. ( jainknowledge.com)

In simple words: it marks the day of Lord Sumatinath’s initiation into renunciation. In Jain tradition, diksha is the formal taking of the path of detachment, discipline, and spiritual practice. ( jainknowledge.com)

If you want, I can also explain the difference between Janma Kalyanak, Diksha Kalyanak, Kevaljnan Kalyanak, and Nirvan Kalyanak in Jain terms.