I don't eat sweets during the week, but I let myself off the hook on Saturdays and Sundays.
ーI allow myself to eat sweets on the weekend.
「週末は甘いものを食べるのを自分に許している」
to allow oneself to ... で「…することを自分に許す」
ーI don't eat sweets during the week, but I let myself off the hook on Saturdays and Sundays.
「平日は甘いものを食べないが、土曜と日曜は大目に見て自分に許している」
during the week で「平日」
to let oneself off the hook で「人を大目にみる・許す・見逃す」
ご参考まで!