Date & Time functions

FIRST.MONDAY

About

Returns the first Monday of the month.

Code

M.S. Excel 
FIRST.MONDAY = LAMBDA(anydate,
    LET(
        start_of_month, DATE(YEAR(anydate), MONTH(anydate), 1),
        CHOOSE(WEEKDAY(start_of_month), 1, 0, 6, 5, 4, 3, 2) + start_of_month
    )
);