Could it be devout Catholic neighbors cooking fish for late Friday suppers? Maybe they don't want to own up to it, though I can't imagine why.

I've read reports of fishy odors associated with electrical problems, but I've never experienced such, and I've run into a few overheating splices and the like in my time.

My parents are very friendly with the neighbours so I couldn't see a reason they would lie.

I've gone around the area which the smell occurs and checked all of the fixtures and sockets and they all seem to be fine. I would have expected the smell to be much more frequent if it was an electrical problem. The pattern of when it occurs just throws me off.

Have the wiring checked anyway. If the house has aluminum wiring, it very well could be a dangerous wiring problem.

So it appears it only happens on weekends. And not every week. My first thought was that it is connected to something that happens with the HVAC or other house systems. You've ruled out anything different at that time of night on those nights? Do they do laundry, use the hot tub, turn on the attic fan, does Mom curl her hair, does the water softener go through its regeneration cycle, anything like that?

Does it happen in winter or only in warm weather?

Weird.

Could the neighbors or a kid of theirs like, ahem, smoking? Probably not but I’ve lived in apartments where that occurred so wanted to throw it out there

I'd look into what roccouple suggested. Weekends and 10 pm. Could be someone smoking some bad weed.

So, I guess the OP posted and ran... never to be heard from again!