The Swiss Calendar

I have recently travelled on Swiss Trains. Not an exceptional activity by itself, though I did use a Swiss Transfer Ticket, available only to foreigners, and valid for two journeys in one month.

As my first journey was the 6th of April, I filled in the travel document on the day. The form is arranged vertically, so:
Day 6
Month 4

When my ticket was inspected, I usually get a stamp, but not this time. The inspector gets out her pen. She carefully inks a 0 (zero) in front of the 6. This I can understand, perhaps I might feel I wanted another outward journey on the 16th, or maybe the 26th of the month, thereby potentially cheating Swiss Rail of some income.

She does the same on the Month line in front of the 4 (April). How many other months can you name utilising 4 in the calendar? Take your time.......

Answers on a postcard :D