If your interview was on a Friday afternoon, are you supposed to send a thank you email on Monday or during the weekend?

First off, this is a solid question; good on you bud! The details matter; strong written communication skills matters; and taking the time/showing you know how to write a proper letter fucking matters. Of those, if you can put together a presentable/passable hand-written thank you note and mail it so it actually ends up getting delivered to the correct address, you have found an edge that is ignored by the overwhelming majority of people these days. Re: timing of that email, depending on how late in the afternoon it is, you could make a case for Monday AM; I also know people who send emails over the weekend - to whit, I always consider those to be a case-by-case basis judgement call based on the vibe/tone you pick up from them as to whether or not that move would be well received; but, personally, I prefer to send thank you notes (email and handwritten) as soon as humanly possible. I find that demonstrates you care, are enthusiastic, and you get shit done in a timely manner/don't fuck around or procrastinate/waste time. Ranking the three, I'd say: 1 ASAP, 2 Monday am (crack of ass early), 3 over the weekend. Obviously, these are my thoughts, not gospel truth, others might have different sentiment(s) on best practices here. Hope this helps!