Sforno on Exodus 12:2/ספורנו על שמות י״ב:ב׳

1החודש הזה לכם ראש חדשים, from now on these months will be yours, to do with as you like. [you have My authority to organise your own calendar. Ed.] This is by way of contrast to the years when you were enslaved when you had no control over your time or timetable at all. [Freedom, i.e. retirement from the “rat race,” means being able to formulate one’s own timetable. Ed.] While you were enslaved, your days, hours, minutes even, were always at the beck and call of your taskmasters.