If you mean Dark Spells and Japanese bundles ... it was slightly predicted, because HH-Games did HH only once from like 5 bundles so far and Japanese was more expensive, these have HH only once in a while.

It seems to happen more often, although if the price is higher then average, then you can usually expect it not to have a Happy hour.

https://steamcommunity.com/groups/groupbuys/discussions

Whenever a new indiegala is announced, the people who organize them make a thread there, and then you can say i want one if you are on time and they got a spot (do mind what region they are from and you, else you might end up paying extra paypal fees).
You can also be a permanent subscriber then you get first on a reservation list, you are free to pass on some bundles, just not too many else it becomes problematic for the hosts.

If there has been a happy hour then they will announce how much it cost, and then you send them the money (usually through paypal) some accept tf2/cs:go keys. And then they will send you the bundle through email.
Some hosts have quit unfortunately, now i am a subscriber with prinzvonbillair, very nice person and she even gives a nudge if you need to pay (which i usually would just forget).

In this way most bundles will cost 80 cents to 1 euro. However since a few months not every bundle will get a happy hour anymore and thus also no groupbuy (then you just don't pay anything and miss out on the bundle) if a bundle is $6-8 then you can atleast safely assume that will never get a happy hour.

They sent me a new key. Duplicate also.

Very unhappy :(