New battery, but still having trouble starting car

Just got a new battery for my 2002 Honda Odyssey on Friday. I did not drive the car Saturday or Sunday, Monday morning it faintly sounded like it would start but then would start clicking. After several attempts it started. I drove about 3 miles to drop my kids off at school and it started right up when I left the school. Should I take it back to where I got my battery or will it be fine if I don’t let it sit for days without starting?

Does the motor turn slowly or is it still clicking?
The cables from the battery to the starter and ground may need to be cleaned.

It turns slowly.

A good, healthy battery should be able to sit for a long time and still have no trouble starting the car. You may have needed a new battery - but its apparent that it wasn’t all you needed. So yes, you have to take it back in.

You need to get the charging system tested.
A basic check is to measure battery voltage with the engine idling and all accessories off. Should be 13.5-14.5V
Turn on headlights and blower. Should stay above 13V.

It is also possible that the new battery is defective. I had this happen once. As a test, I disconnected the battery and brought it up with a battery charger. I measured the voltage after I charged the battery and it measured 12.6 volts as it should. I left the battery cables disconnected and measured the voltage 3 hours later. It was just over 9 volts. I returned to the store where I bought the battery and they didn’t have a replacement, so I got my money back and went to another store. The new battery solved the problem.
At any rate, an electrical system check is in order to be certain whether it is the alternator, the battery, or something pulling power when the car is parked.

Did this begin with the new battery? If yes I’d take it back and tell them you can’t have this problem with kids in the van and have them consider the previous reply of a defective battery. And it’s possible you have a drain of power somewhere that you’re not aware of like a light left on in the back -glovebox?, a stuck relay/switch, or a small short. Diagnosing those could be minutes to hours. Until you find out why I’d keep tabs on it by having the battery tested and charged up frequently so you don’t get stranded with your kids.

The trouble very well could be due to a weak charging system or a excessive current drain on the battery while the car is parked. Have those things checked out at a shop. Have them do a load test on the charging system and check the battery current drain while parked.

Is it too hard to imagine that a battery is just a bucket that can be filled with current and when it is poured out it must be refilled? A new bucket may look pretty and shouldn’t leak but it cannot refill itself.