Asklemmy

This is actually a HUGE factor in human sexual selection. Other primates have visible Estrous cycles. I.e. what people commonly call going into heat.

This has a big impact on reproductive dynamics. Who mates, when they mate, etc.

Humans, for whatever reason, do not have a visible indicator of fertility, which provides females more sexual selection options. Because it's not obvious when they're in heat, they can choose who they mate with at the right time. They can also socially mate all the time, which can increase human bonding.

From what I have heard/been told the "ignoring partner and sex 1-2 times a year" is a lot of marriages, so maybe your scenario wouldn't change much

It would be like the purge, but stickier.

We sort of do

Reminds me of how the Vulkans have it in Star Trek. I'm no expert but there was an episode where Spock went into his horny craze :)

first two things come to mind:

water-cooler conversations at work would be different. telecommuting may be an important option during the season.

My shiny seashell collection may be in jeopardy. the ladies love seashells 🐚.

It would be a very large undertaking to describe the changes since hidden breeding cycles are one of the things that separates humans from other apes.

Human women being able to choose when and with whom to mate, secretly, is one of the most significant aspects of what it means to be human.

Humanity is one of the few species whose evolution is primarily driven by sexual selection and not just the “I survived long enough to find a female so I reproduced” strategy we know of as natural selection.

I imagine it would look a lot like how Zoidberg’s home world is depicted on Futurama

Post nut clarity for most of the year sounds peaceful, but I'm guessing people in heat would think with their genitals and wouldn't care about STDs or pregnancy.

Everyone wound have some STD and birth control would be a side note. Human reproduction would work like clockwork and without neutering world would have exploded long time ago.

We sortof do, there's a huge cluster of birthdays roughly july to september

I had the discussion the other day of how civilization would be different if humans followed the 'have loads of babies at once and see which ones survive' style of reproduction.

"Oh hi Sarah! How're the kids?"

"Oh, little Jeremy wasn't eating as much as the others so I threw him outside onto the road."

Does it happen en masse or does each woman have their own heat season? Say, does everyone go crazy during the same 2-3 weeks or does it vary per person like it happens with periods?

In the masse heat I think there would be some obvious differences in stuff like festivities and each culture would adapt to these seasons. There would be a need to gather and store resources before the rut, and also to be prepared for the inevitable birthing en masse to come. We would probably have public holidays for the mating seasons and festivities for birthing season. In contemporary times, hospitals would need to do some heavy work planning and preparing maternity ward space to accommodate the seasonal influx. That's just what pops to mind.

Not too sure about the differences in the second case though. One option is, women would be allowed to retreat from society and become hermits during heat if they want to avoid pregnancy and this would be totally normal. Or, some cultures would lock them to prevent them getting pregnant. Coming to think of it, I think having women randomly in some kind of irresistible heat like this would be very disruptive and impossible to prepare for so I don't see much happening in the way of gender equality and women's rights sadly. I'm inclined to think the re would be a lot of gender based extreme segregation.

I'll point out that many creatures that have seasonal mating still mate for life.