Mil or even milieu de semaine wouldn’t be understood in France if you mean the period from Monday to Thursday. One of the reasons is that period of time is not considered to be in the middle of the week by the majority of French people, the first day of the week being Monday for the most of us. Milieu de semaine means Wednesday or around Wednesday (Tuesday to Thursday) in French.

You might use:

lun-ven | sam | dim

or

semaine | sam | dim

An official name for the five working days is jours ouvrés but semaine (especially in the expression en semaine) is common.

Yeah, “during the week”, “in the week” or just “the week” are most common in English (UK) too …