Does it matter when you send your client an invoice? Of course, you should have an agreement in place before you start working, but what should it be?
A common recommendation is to bill every 2 weeks, but many bill monthly. For me, the frequent billing were adding extra overhead and the schedule was hard to keep track of so I went with monthly.
Should you bill on the last day of the week or month, or on the first? I settled on the first, so I don't have to scramble to do the accounting after a day of work.