I'm pretty new to steam and was exited to find out this weekend Steam had the 2k catalog discounted.
I bought Bioshock 2 on the first day as it was 50% off but then found they discounted it even more the next day. In this case I wasn't to fussed as it was only a couple of $.
Is this something they do often? In future should I wait till near the end of the 'promo period' before buying to get the best deal.
Sometime they do such kind of discounts but not so often. My suggestion is ... well what i am doing that i add games on wishlist (you will get a notification if the game is on sale) and wait for bigger steam sales like Summer Sales or Holiday (Christmas) Sales where are games for most of the time much cheaper, especially between Flash Deals (deals that change every 8 hours).
I am also suggesting to you that you limit yourself with the money for ever game. My budget for every game on sale is 5€ to 10€ because i don't have a lot of money.
So to wrap up, just keep eyes peeled on good steam deals :)
they do that constantly, usisoft had one of those weekends, i spent around $50 on the assassins creed series, the next day when it was assassins creed day i would've pair around half that, if a publishers whole catalog is on sale wait untill the day that the franchise you want is the headliner