Look at what the competition is doing.
I also want double points day!!!!!1!!11!
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The points on SO are calculated according to your past actions - and until now they can be recalculated any time.
That means that implementing a "double points day" would require a specific rule such as "x2 multiplier for actions made on that day" - thus noticeably complicating the reputation system. (And in another, more significant instance, they already refused to introduce a time-dependant rule for calculating reputation in order to grandfather already earned points).
Also, what purpose would that serve? The competing sites do that in order to encourage people to write more (good) answers. Is the problem on SO a lack of answers? I don't think it is.