It's habit. At a certain time you used to always do something, and now you don't.
The best way to deal with it is to get a new habit that occurs at the same time but is unable to coexist with the old habit.
So I guess you could play a sport, like tennis, or cook a big meal. Anything incompatible with going by the liquor store.
You'll either pick up a new good habit, or by the time you get bored of it be significantly less cognitively attached to the idea of going to buy booze on Sundays.
Hope that helps.