Translation of ~のことだ?


I was wondering how I should or could translate "no koto da" as in

> 僕が六歳だったとき **のことだ** 。?

I get the general meaning of the sentence, but I have no idea what the meaning
of こと (koto) in this case is.




The Little Prince?

I can't give you a grammatical explanation but it's the difference between:

> 1) 僕が六歳だったとき...すごい絵を見た。  
>  When I was six I saw an amazing picture ...  
>  2) 僕が六歳だったときのことだ。...すごい絵を見た。  
>  It was when I was six. I saw an amazing picture...

Unfortunately I'm having a hard time describing the difference between my
English translations. I suppose in 2) 僕が六歳だったときのことだ is setting the scene for
everything that comes afterwards. Whereas 1) is just a boring statement that
the boy saw a picture when he was six.

