I get many emails from multiple online shops and forums and whatnot on my birthday. Some of them include a voucher to buy something with a discount.
I just had a funny thought about a different kind of birthday attack: register a bunch of accounts with different birthdays to all the online shops you find worth it and wait for nice vouchers to drop one by one.
The original, actual birthday attack is very fascinating and hugely against one’s intuition. Check it out if you’re not familiar with it.