Some thing is draining your battery. See if you can get hold of a clip-on ammeter (most accessory shops sell them quite cheaply) and connect it to the thick battery main output cable. Make sure everything is switched off and the car is as you would leave it (including the alarm set if you have one). The reading on the ammeter should be minimal. If it is 1 amp or more pullout and replace the fuses out 1 by 1 until the reading drops to zero. Then you will know which circuit is causing the problem, then you can check for silly things like courtesy light switches etc.
If you have an after- market alarm/immobiliser fitted, pull the fuse on that first, you may well find that that is the problem. The you’ll have the delightful task of trying to figure out someone else’s wiring. You may find that removing it and staring again is the best option.