I think a weekend or holiday "day" is probably 8 am - 8 pm, so splitting the day would be an exchange at 2 pm.
School day birthdays - school lets out at 2 pm, so the "shifts" would be 2-5 and 5-8.
Holiday shifts will probably be ranked in order of importance so each parent gets an equal "quality" of shifts every year - ie: for many the most important portions of the holiday would be Christmas morning, NYE, Easter morning, etc etc.. Nobody gets all the best shifts, should be balanced.
During periods of no travel during school breaks, the regular schedule should apply for stability and continuity's sake. Any travel should be planned for days when that parent already has custody under the regular schedule to avoid too much modification to the schedule before/after the travel.
Either parent can attend sports games, practices, lessons, extracurriculars, etc. Based on my son's schedule, attending events plus my days of custody would provide an opportunity to see him at least 5 days a week, sometimes 6. Not bad.
I'm staying positive, silver linings, Daddy runs the show at the house without interruption or critique, that will be nice. But how much is the back and forth going to suck for the kids? Worried about this, but it's happening no matter. Love input on what has/hasn't worked for others.