A calendar with 13 months of 28 days each. This would result in a year with 364 days. However, it wouldn't align with the solar year, which is about 365.24 days. The extra .24 days accumulate to create the need for a leap year.
Simply put, this system would simplify the calendar but would still require some kind of adjustment to stay in sync with the seasons.
Aka it wouldn't work.