Any day of the week is fine for having sex as long as you;
1. Don't do it where it will cause someone else pain or embarrassment
2. Don't use it as a "weapon" or "reward".
3. Have a partner who is "Ready, Willing and Able" when you are, don't ever force sex on your parntner when the person says "No" or "Later" you might be without that person if you do those things.
I had a girldfriend thjat always wanted sex and there were times when I just wasn't in the mood and I told her so once and she got mad a left...Then got a partner who is thoughtful, caring and has made sex fun for me again.