when i was boy,the most exciting thing was when to celebrate

when i was boy,the most exciting thing was when to celebrate the Spring Festival.
WAS后为什么要加A?为什么不去掉TO,而是WHEN?
and every year more and more people start a stamp collection of their own and discover an interest which can even last lifetime.last前加A，有什么用？
介词后什么情况下加冠词？
as we were all left home at early age.early前加AN可不可以理解为介词后加不定冠词呢？这句话还有毛病吧？