On the other hand the calendar is always the same, years always start on a Monday, months start on a Monday
You could have a permanent calendar, except that religious holidays and astronomical events (solstices, equinoxes, best day for planting tulips) would move around
My birthday would vanish as it’s towards the end of one of the middle months; my mother’s birthday would only happen in leap years as it’s late in the last month
Birthdays on the new calendar would always fall on the same week day (though people born under the old calendar could celebrate on the day that it would be had the old calendar continued, or choose the same day number and month, or choose the corresponding day in the first year (or whatever) of the new calendar and stick to that month and day number)