Translation:What is the thirteenth month in the Chinese calendar?
Mange takk for svaret ditt. Det hadde jeg ikke tenkt på. Hver dag lærer jeg noe nytt om norsk.
It's not the Chinese calendar but there's a thirteenth month in a proposed but never adopted calendar called 'Sol'
It is, indeed, The Chinese Calendar.
"To prevent the lunar calendar from becoming more than half a month out of sync with the solar calendar, an extra 'leap month' is added in the Chinese calendar every 32 or 33 months. So every second or third Chinese calendar year has 13 months and 383–385 days."