The way I originally read Dutch's question (and Dutch can feel free to comment on my interpretation) was -
Earth based individuals relocate to a planet which has 668 days per year, instead of 365. How would they handle holidays?
Two ways that I see -
1 - Lock Down to the Earth Calendar
Establish the same calendar as Earth (allowing the day cycle to be local), but the monthly and annual calendars to be Earth based. This way even though it may seem to locals like you are celebrating your Earth holidays twice per year (in year 1 you celebrate Christmas on day 358, but in year 2 you celebrate it on day 48 and then again on day 406). This is likely to annoy locals, but would be used if there is still considerable communication back to Earth.
2 - Adapt the calendar to Local Annual Calendar
Insert a 25 day month between every Earth Calendar month, and then add 3 extra calendar days at appropriate points in the year related to local astronomical conditions. Christmas would then still occur on December 25, but instead of being day 358 in the calendar, it becomes day 661.
Keep in mind that unless you pick a single universal representation of time -
ex: 1 "Earth Year" = 8,760 hours = 525,600 minutes = 31,536,000 seconds
Any discussion of time, ages, dates, etc. becomes extremely confusing to individuals from different planets/solar systems.