Semaine

Cet officier est de semaine = He is officer of the week.

Il dépensa toute sa semaine = He spent all his week's wages (or, pocket-money).

Je le ferai la semaine des trois (quatre) jeudis = I shall do it in a week of Sundays (i.e. never).

[Also: Je le ferai quand les poules auront des dents.]

Prêter à la petite semaine = To lend money at high interest for a short time.