The format description FM which toggles Fill Mode for TO_CHAR when used to dates is not recognized. For example:

SELECT CONCAT('/', TO_CHAR('2020-01-06 10:11:12', 'DAY'), '/');

Returns (note two spces at the end to fill up to the length of the longest day name (Wed):

/Monday  /

The FM flag in Oracle negates this blank filling:

SELECT CONCAT('/', TO_CHAR('2020-01-06 10:11:12', 'FMDAY'), '/');

Returns:

/Monday/

In MariaDB the result of the latter is an error:
ERROR 3047 (HY000): Invalid argument error: date format not recognized at FMDAY in function to_char.