I understand that they can't write letters for the first few weeks or something like that. So what week (week 1, week 2, week 3, etc.) are they allowed to start writing letters home? And is it only on Sundays they can send them out?

Thanks in advance. My man left on Wednesday, so it's weird not being in contact with him. We used to talk every day, so I'm so anxious for the first letter.
I get so confused. Some sources say they can't write at all for the first 2 - 3 weeks, while other sources say they are allowed to write within the very first week. I guess I'll find out when I get my first letter. I can't wait. When I see it it'll be like a Christmas present to me haha.