It's related to the last allotment day. I don't think we know what it is yet.
For the estimates, it is as follows:
...no later than three (3) sitting days before the final sitting of the supply period ending March 26 (Monday, March 21, 2016) or three sitting days before the last allotted day in the current period (which has not been allotted yet).
We don't know when the last allotted day is.