The Mouvelian calender has 360 days spread over 4 'seasons' of 90 days each, whereas the Gregorian calender has 365 1/4 days spread over 12 months with a varying amount of days. So to make this all work, the most logical solution (presented by Inasis) seemed to fit those 5 extra days (and a 6th for every leap year) into the 360 days of the Mouvelian calender.
The Season of the Zephyr was easy enough, as January, February and March have exactly 90 days in every normal year, so the only adaptation was to include every 29th of February into the same in-game day that is normally the 28th of February (being the 59th of the Zephyr).
For the Seasons of the Phoenix, Scion and Colossus, I had to turn the 30th and 31st of every month that have 31 days into the same in-game day, to make the whole thing fit (and not invent extra days that the Mouvelian calender doesn't have).
ArenaNet recently adapted the Mouvelian calendar and gave it as much days as the Gregorian calendar (apart from that 1/4 day every year that causes the leap year cycle), however, and the lovely Inasis Nassimi took the time to create new charts that integrate that recent change, where I didn't have the time to do it myself. My old charts can be found in the spoiler thingy and below that the wonderful new charts by Inasis.
Spoiler: Old ChartsShow
Spoiler: 1327 (Previous Year)Show