The context of the phrase:
Participants are not allowed to come late. In such case they just don’t come. When you arrive, you will find the next meeting is next Friday (which datum it might announce on the notice board or something like that).
I assume that the definition of it in this phrase means
And the meaning of the phrase then becomes: “which datum might be announced on the notice board.”
Is it an irregular/spoken use or does it appear in literature?