Advice regarding daily sale variations

I understand that there isn’t going to be any guarantees when selling on Amazon, but the variation in sales seems to have no pattern at all.

Logically, I would have thought that sales would increase around when people get paid at the end of the month and then decrease slowly throughout the month and there would be specific days and times of days. I would be more likely to make sales but this hasn’t been the case at all.

Some days I can make 40 and then the next day 200 in sales, I also have quite a lot of occurrences where I won’t sell a particular item for weeks and then two people will buy it within 5 minutes of each other.

I don’t use any advertising so all sales come organically but the whole thing just confuses me. Is this a regular thing but other sellers?

This is a side hustle for me. And I am an FBM merchant. I have 12 months of sales data.