The tooltip for Selebrak says simply "Some seasonal effects are boosted", which is very vague.
The Wikia however seems to have been updated, and gives the effect of Selebrak according to the various seasons. The interesting thing about Valentine's Day is that this seems to be the only one where the beneficial effect of Selebrak (heart cookies more powerful) persists even when Valentine's is not current. that is, heart cookies acquired while Selebrak is slotted give a bigger CpS boost that otherwise, and this boost persists even after Valentine's is terminated. At least that's what the Wikia seems to imply, and that is borne out by my game. I had been trying to track down why this particular run was producing about 7% more cookies than I was calculating, and I think it's because I had Selebrak slotted while in Valentine's season.
It's not a massive boost - but still, a few % is not to be sneezed at, so the message is that if you have worship swaps to spare it may be worth putting Selebrak in a slot before purchasing any heart cookies.