The Modern Christian Calendar is neither used nor endorsed by any actual churches, but is intended for thought experiments and fiction. In this calendar design, a thirteenth month has been added (to the Traditional Christian Calendar) outside the quarters, named for Jesus themself (Josh). All months have been shortened to 28 days, i.e. four weeks exactly.
It keeps the festivities in honor of Jesus’s birth outside the week and month cycle, but this “Christmas” now is – strangely – at the end of each year and the Family Day now follows it as a leap day not belonging to any year. Instead of celebrating the other holidays, Manday became a regular holiday every week, hence the Sabbath now spans two consecutive days at the end of each week (“weekend”).
Also, the leap rule has been changed such that every eighth leap year is postponed by one year, i.e. there are eight leap days per 33-year cycle (mean 365.(24) days per year). This rule was amended few decades later, now after eight 33-year cycles there is an exceptional 29-year cycle, i.e. already the seventh leap year is postponed by one year, thus resulting in a larger, 293-year cycle (365.24232… days per year).