I have always thought/known that Mad Sunday is the First in June, but maybe that's about to change?
I cant find any dates, but, on the IOM Government website it gives these dates for the school holidays -
Half term (TT)                  Monday 2nd June 2025 to Friday 6th June 2025                    Tuesday 26th May 2026 to Friday 5th June 2026
                                                           (this year)                                                                             (next year)
Seems like a longer holiday next year, but still earlier than anticipated?

Also strange how I cant find any indication as to when boat tickets go on sale this year, for next year!
Maybe they really haven't decided when it's happening next year!!! (I'm presuming 19th May!)