Trying for another

My husband and I decided we're gonna try our last baby but it's so hard cuz he has to go to bed early cuz he has to be up at like 3 n the morning for work Monday through Friday so we only try on Friday and Saturday night even then it's not till after 3 or 4 and sometimes it still doesn't happen cuz my 2 year old takes forever to fall asleep and my 8 year old fights to stay up on the weekend. Any tips?