Hi Marianne
So now that you have found an Egyptian day 1 psḏntyw festival, do you still hold that the celebration mentioned at Megiddo was a purely Canaanite celebration? If so bear in mind that, from what we know about the Canaanites, their emphasis was on the evening sighting of new crescent (ḫdš) not the dawn non-sighting of the old crescent. Normally one might expect that, in the Egyptian calendar, the earliest (and most common) sighting of the new crescent would occur on the evening of Egyptian day 2, a day that began with the morning and which the Egyptians called tp ꜣbd, “the head of the month (new crescent”. (If not sighted on this evening it would be seen on the evening of Egyptian day 3 which they called mspr, “arrival”).
So if your interpretation is correct than the phrase refers to an otherwise unknown Canaanite festival that contradicts what we know about the calendar of the Canaanites.
Regards Joe