His point is that his AvA event was scheduled based on the the proposed Scenario date to avoid a conflict. Because the scenario is being pushed back, a conflict now exists. Maybe you should coordinate with each other as a courtesy.
As mentioned above, the scenario date isn't necessarily pushed forward. We'll see.
As for advance notice, here is the scenario schedule for the next N years:
Scenario in February, June, October.
Now you know it as far in advance as you want.
However, the exact date is subject to variation because scenarios take a lot of production and are subject to timings that can't be controlled in advance.
This is why I say things like "tentatively scheduled for February".
