Because I honestly don't think I am? I arranged the party, ergo the work should be all mine according to him. According to him he wasn't aware we were having a party. The three year old and 6 year old have talked of nothing else for weeks, but he didn't know about it 
Only I am at work all day the day before and the morning of the party. He is off both days and my mum has the children all day the day before.
I mean, honestly, is it too much to ask? I will do the food/shopping/hosting etc. All I want is for him to decorate the room.