I had 2 offers at the time. For one of these it took ages, almost a month. But for the other one, I received the answer straight after the interview and the letter arrived within a few days. I guess it all depends on the workload of the academic and admin staff. I wouldn't worry about how it looks. It is your right to call and find out. But as you said, leave it to Monday. Good luck, I hope the result is positive.