Yes, definitely plan for flexibility.
I-Day has moved back & forth over the years. In my day, it started after July 4 (the 6th to be exact). Then it moved to the last week in June. In very recent years, it has been right before July 4. The Supe has some (if not all) discretion and there will soon be a new Supe (due to normal rotation), although maybe not before I-Day. So, plan on the 1st but don't be surprised if it changes.