02 Cavalier Won't start/turn over

Recently replaced battery/alternator. Car ran great for a week. Then one
morning it wouldn't turn over or start. No interior lights or dash lights would
come on and the key fob wouldn't work to lock the doors. The key is not
chipped, so I don't think that's a concern. I checked the battery cables and
they were tight. Went back down after about an hour and it started right up.
Had voltage tested and it was fine. Another week goes by. Car sits for a day.  I
go to start it and the same thing happens, only this time it's a day later and it
still won't start. No lights. No nothing. Any ideas?