http://dictionary.cambridge.org/grammar/british-grammar/pronouns/it
In this link, I found the sentence '' It is August that you are going on holiday''.
I wonder why it is not ''in August''? I think the sentence '' It is in August that you are going on holiday'' will be sound.
The second question is ''Can I use where/when in it-cleft sentence?''
E.g, It was the market where I met her.