There’s a chance that the battery is old enough that it won’t hold charge for long, so a check of the battery’s general health would be the first step here. Beyond that, conventional wisdom holds that there’s something in the car that’s staying on even though the ignition switch is off, and that’s what’s draining the battery. But before drawing the latter conclusion, attend to the basics: Make sure the battery terminals are clean and tight and test the vehicle with the engine running to make sure the alternator is, in fact, charging the battery at the correct rate. Most tradesmen agree that something between 13.5 and 14.5 volts at a fast idle is about right for the alternator. While the voltmeter is hooked up, turn on the headlights and make sure that the alternator keeps up. If the voltage drops during this stress test, you could have a dodgy regulator. If that all checks out, the usual suspects here become a stereo (particularly an aftermarket one) or an alarm system (ditto) that is draining the battery. Make sure that the ignition switch is, indeed, turning everything off and then go back and check the car in the dark to see if there’s a courtesy light or underbonnet light that’s still on and slowly sending the battery flat. If nothing sticks out as being wrong, the next step would be to take to the car to an auto electrician who can use a multi-meter to check each circuit in the car individually until they find the one that’s energised when it shouldn’t be. It doesn’t take a huge current draw to flatten a battery or at least take it to the point where it will no longer start the car. Ignoring this will not only eventually leave you stranded, it will send your battery to an early grave as batteries don’t appreciate being flattened over and over again.