If you already know the people in mind you'd like to write a LOR, I'd recommend the first week back if you're still in school. Otherwise, those people are going to start receiving a stack of LOR requests as time marches on in the semester.
Also, I'd recommend making them a folder with your cv, personal statement (or at least your most up to date draft), as well as a cover letter with instructions.
don't be cheap with them and just casually ask them for a LOR unless you know them really well and/or they're practically your buddy. Even then, it's far more professional to go the route above. Don't spend zero time putting your request together and then expect the faculty member and/or person to spend several hours writing a letter to get you in.