Facts due to movements of earth and moon:
- 1 solar year = about 365.25 days
- 1 lunar month = about 29.5 days
The leap year rules from the Gregorian calendar are good because they keep the calendar year well aligned with the solar year.
But the weird lengths of the months and shifting week day of January 1st are drawbacks in many respects.
It would be so much nicer to have a perennial calendar so that every year begins on the same week day and the week day of any given day in a month is predictable.
For these and many other reasons, I like the proposed World Calendar.