Calories

It depends. Your body doesn't reset at midnight every night. So, if you know that you're going to be doing something on... say, Sunday, and you choose to go under your daily calories by 150 from Monday to Saturday, come Sunday, you have 900 calories (150x6) on top of Sunday's allotment for a splurge.

In general, unless you're very short very sedentary, you should not be under 1200 calories for a woman, 1500 for a man.

Go too far below your recommended calories and, while initially you might be okay, over time a steep deficit will catch up and you're likely to be hungry, lethargic, etc.

That being said, if you don't hit your calories precisely, day in, day out, it's fine. One day where you're way under or over won't make a difference in the long run. (The scale might give you a heads-up if you weigh in the day after a splurge, just like it will if you weigh-in while significantly dehydrated. But it'll even out over the next few days.)

One of the biggest benefits to not eating all of your calories allowed is to account for errors in logging. Depending on how you are weighing your food or logging it you can have errors. Ensuring you are a bit under helps make sure that you are actually not going over. This can be especially true when logging foods when eating out where weighing foods is not normally the rule. For instance, I may weigh every item to the gram/ml in my meals at home, but when I go out to a salad bar or AYCE Asian Buffet, I just estimate it and try to log high.