[antognini]

It's not so hard to know if there will be an extra month using the Metonic cycle. You just add an extra month in the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years.

That may sound complicated to remember, but consider how we figure out how long a month is in the Gregorian calendar. There are 30 days in the 4th, 6th, 9th, and 11th months and 28 days in the 2nd month (except for leap years). At least with the old lunisolar calendar you can figure out where you are in a month just by looking at the moon.