Double & Triple SP Events

Info
Double SP Weekend is a promotional event that rewards players with twice the normal SP towards Unit Ranks.

The first of this event was introduced on October 26th, 2012.

Official Announcement - 10/26/2012
Your troops have been putting in a lot of extra time lately with all the Infected running rampant around the Outpost. They deserve a little ego boost, so we’re dishing out Double SP this weekend! Send them back to the front lines to defeat enemies. Their bravery (and survival) will be rewarded with promotions to higher ranks.

Cheers!

-- Ikki

Source: http://battlenations.com/forums/topic/1026-1029-double-sp-weekend/

Effect
Quite simply, anytime SP is rewarded, the player is instead rewarded twice the usual amount (which is then distributed amongst the participating units in battle, as normal). This boost can be combined with the purchasable double-SP boosts, for quadruple the normal rewards!