See https://www.jevents.net/frequently-asked-questions/custom-overrides-of-language-strings and the translations of

JEV_SUNDAY_CHR, JEV_MONDAY_CHR etc.

Thanks you very much for introduce , I share how to edit day name of j 3.2.21 mini calendar of 1 letter to 3 letter (S to Sun).

1. Login to Joomla Administrator and cursor to Extensions -> click to Language Manager.
2. Click to Overrides.
3. Click to + New and add translations of JEV_SUNDAY_CHR -> Sun / JEV_MONDAY_CHR -> Mon / JEV_TUESDAY_CHR -> Tue / JEV_WEDNESDAY_CHR -> Wed / JEV_THURSDAY_CHR -> Thu / JEV_FRIDAY_CHR -> Fri / JEV_SATURDAY_CHR -> Sat and complete to edit day name of j 3.2.21 mini calendar of 1 letter to 3 letter.