What's meant by the expression 'zero hour' as applied to legislature sessions?

Answer 1

The zero hour is the time of the vote. So something happening at the zero hour is happening just before the final vote when there's no or little time left.

Answer 2

It refers to the Question Hour. During this period, a member can post questions to any minister of the goverment on any subject. However for smooth functioning of this, any meber wanting to pose a question will be required to submit his wish to the speaker(ithink the day before, not sure). The speaker then decides on ehich members to call. The called members can pose their question.