Disadvantages
- For the superstitious, a disadvantage to this format is that every month includes a Friday the 13th, and this date occurs thirteen times every year. This is readily solved by making the 1st a Monday.
- Thirteen, being prime, is not evenly divisible, putting all activities currently done on a quarterly basis out of alignment with the months; each quarter would be 13 weeks instead.
- Several religious groups oppose any interruption of the seven-weekday sequence.
Read more about this topic: International Fixed Calendar