There are many factors that contribute for price variation, such as hours of the day, TOTW and launch of the new FIFA series. But Happy Hours during FUTmas are the most effevtive one. Hundreds of thousands of players will buy and sell packs. The natural relation between supply and demand will change. Thus, the banlance of market is disrupted. both cards and coins circulation increases on the transfer market.
What we see most appearently and effectively is the immediate fall and rise of the price. FIFA 15 coins price will keep low for a while when the Happy Hour is moving on. But back to normal and what it was before ends of the hour with the market instantly absorbing the addictional cards. It can be said that pirces are affected and changeable throughout the entire Happy Hours. Comparing to the price that is supposed to be if there will be no Happy Hours, the price that effectively jumps after one usually ups 3% to 5% value of the card.
EA launches more and more Happy Hours every year. While Happy Hour packs including 35k, 50k and 100k are the most popuplar. The bigger the pack is, the sharpper drop of the price is.
Notice how after the Happy Hours end the price is inferior to what it would be if a Happy Hour never happened while the natural tendency for it would be to stay the same.
Happy Hours will not be announced with antecedence for what the market can’t really react with such.