Why 1992 Olds 88 Royale 3.8L starts fine for 5 days then day 6 turns over fine but won't start then back fires Day 7 I charged batteries Day 8-12 Car started like normal day 13 same as day 6?

While I am no means an expert. I would say check your coil Packs & the ignition starter which is located under the coil packs. There are 3 coil packs. Or your crank sensor could be going. If you notice your car stalling out even on short trips & not wanting to start back up, till the car cools off. I would say crank sensor. I have had to replace all of these things in my car, plus a water pump.