Last year FIFA added a whopping 23 fan favourite players of previous FIFA series, including David Luiz, Torres and Eto'o. These players are not the normal you are used to, but boosted players with extra good statistics. To get these kind of players you need to open FIFA packs while the event is ongoing or buy special FIFA birthday packs. Also this year we can expect the birthday squad, but wich players will be in the squad will still remain the question.
Last year you could complete Premium SBC's to increase your chances to get your hands on premium items, ultimate packs, squad cards, SBC kits and beside that also premium player cards. Last year the premium player cards existed of Fernando Torres, David Luis and El Shaarawy. The players you win through Premium SBC's are even better than the players from the birthday squad! So get yourself ready for the premium squad building challenges. Last year you needed squad ratings of 87, 88 and 89, so prepare yourself for putting some good teams together. You can put together some good teams to prep yourself by buying FIFA Packs.
For the real loyal FIFA players you have daily birthday SBC's. These are easier to complete than the premium SBC's. They run up to 8 challenges in total and after completing 3 challenges you get your first reward. Last year the different divisions looked like this: