We just moved from Southern California to Colorado Springs. My kids missed one day of school. My husband picked them up from school on Thursday afternoon last week and hit the road. They arrived in Colorado Saturday and started school this past Monday morning. So they missed one day - a Friday.
I really wanted it to be a quick, smooth transition. We got them registered ahead of time and made sure all of our ducks were in a row. Another piece was that it's much easier to unpack with them at school.
We moved in October, when DS was in first grade. It worked out in a way that we moved while the new school was on a 2-day break. He had just finished the first 9 weeks at the old school a few days before we moved so he had his grades. Then we spent 2 days moving but the new school had those days off anyway because they had just finished their 9 weeks. He started at the new school the first day of the second 9-week grading period. I recognize we were lucky that my employer allowed some flexibility with dates and the school calendars lined up well. Registration was done ahead of time. ETA: I am sorry, I did not pay attention to the board I was posting on. Good luck with the move.