Hyundai Repair: 2003 Sonata Start up, load test, electronic test

Question
Hello. In February ,I replaced my battery on my '03 Sonata, because my mechanic told me to it before inspection in April. In March I had to have it jumped.  Brought it back to the mechanic who thinks I left the lights on. I told him it was daytime and I hadn't used the car for 3 days and I don't drive at night. He recharges the battery. Yesterday morning I do errands, park my car at home. At 1pm I try and start my car at it won't start. I did not use the lights, or the radio. I call AAA and get it jumped, keep it running for 20 minutes. Shut the car off, go into my home and an hour later go back to the car to start it only to find it won't start.  I have never had any problems with this car. What could be the problem?

Answer
Either your battery is defective or your alternator isn't charging the battery properly.  You've listed more than one instance where the car wasn't off for long enough for items remaining on when the car is off to drain the battery.

Did your mechanic test the battery electronically or apply a load?  I've seen several batteries pass an electronic test only to be unable immediately afterward to start the car.  On the other hand, it's very rare that a battery will pass a load test and still be unable to start the car.