In my opinion, birthday is the most important holiday или one of the most important holidays

It has become fun

I have been waiting fot this

I have an idea how you can celebrate 

IN My opinion, the birthday  IS  ONE of the most important holidays;

It was  funNY;

I have  BEEN waitING for this BIRTHDAY;ИЛИ I have  BEEN waitING for IT

I HAVE AN Idea  how you can celebrate    или

THE Idea of youR  celebratING IS