1) It was yesterday that I met his sister.
In here "it" is used as the impersonal subject of the verb to be, esp. to refer to time, distance, or the weather.
2) Itís my fault as I asked a stupid question.
It's is a contraction of it is. In here "it" is used to represent a person or animal understood, previously mentioned, or about to be mentioned whose gender is unknown or disregarded.