Do we have the same child? Mine does the same, we are having a great day and all of a sudden it's like the wheels fall off the cart. AHHHHH!!! You wonder if maybe you had twins, Dr. Jekyll and Mr. Hyde. HMMMMMMMMM....
Most likely what happened was that since a month ago he may have been receiving a great deal of attention for acting out, and got even more attention for behaving well (positive reinforcement,) that maybe he felt that he was not getting enough attention on Christmas (you and your spouse were unwrapping gifts,) that he figured he was going to get that attention back (good or bad.) At that age, kids cannot distinguish between good or bad attention, all they know is that you are engaging them. It is that engagement that allows them to feel safe and that they are significant. Remember, a child's biggest fear is that of abandonment, so any time that they are acknowledged spoken to or dealt with it is their opportunity to be heard and have their needs met. In no way am I saying anything about abandonment in relation to you, it's just that their survival skills kick in. Also, kids still think concretely at this age and are still learning that just because mommie is not involved with you at this moment, it does not mean she does not love me.
As the old saying goes children should be seen and not heard, well apparently our kids see it the another way, they need to be SEEN and HEARD... ALOT and OFTEN!!
You did the right thing and do not feel guilty. Kids can't think that far along and that insync at that age. They deal with what is in front of them at that moment and no more. It is our job as parents to let them know that the momentary reaction they just had has consequences. That is how they learn, unfortunately, it is a task we must continue to do. Eventually, they will outgrow this phase, your sanity will possibly still be intact and they will know that it's not always about them. For now, they think it is and we cannot feel bad to remind them that it is not.
As for the birthday party, I would suggest that you tell him your concerns and discuss your preparations with him every day as the event approaches. Include him in the preparations and make him aware that his behavior will have a direct impact on the outcome of his big day. Maybe take him to pick out the cake and take a picture of the cake, pick a spot and photograph it, as well (photograph everything you can for the party.) Then take all the pictures for his party put them on a posterboard (title it My birthday party) and in the same way you gave stickers for good behavior, you take down pics of his bday party down for bad behavior. Depending on the offense, take something away (proortionality.) Make sure you set up clear rules and give him a chance to win things back. He will feel invested and may enjoy his party even more if he knows he was such a good boy and played such an important role in making it happen... besides actually being the birthday boy, that is. Good Luck to you and to me, again I am in the same boat.
Again, We do not reward negative behavior, we do as you do, timeout, confiscation, etc. As they continue to receive the same discipline, they will snap to the fact that they need to change their behavior... in order to improve their situation. Again, may sound basic, but it's about survival. Until then be patient... only 14 more years to go.