In this thread it is mentioned that is is 39 days (by empirical testing):

http://discussions.support.ovi.com/t...e/td-p/1535490

The reason is as mysterious as everywhere else...

To add more to the mystery:

Yesterday I added a new calendar called 'Zzz' (manage calendars
behind button in lower right corner and then '+')

I tried all kinds of tricks with this calendar (restarting calendar every
now and then). Finally, after some combinations of restarting and
disabling the 'Zzz' calendar. My synced December calendar entries
started to show.
I restarted the calendar a few times and the entries were still visible.
However, this morning December view was again empty...

Now I tried some combinations of enabling and disabling the 'Zzz' calendar
(without help) and then making 'Zzz' default and taking that default
out -- and then I could see my December entries again...
... but when tried to reproduce I lost those...

I got those back third time -- It was getting harder every time. This time
-- aftet trying a while -- I removed 'Zzz', recreated it, enabled and disabled
it, made it default and took that default off -- and finally after some
combinations of these (restarting calendar at arbitrary times) I got
my December events visible again.

Next time when I loose my December events again (my wild guess is
tomorrow) I'll try to play with 'Zzz' calendar without restarting the
calendar application (and just use [<-] button in lower left corner to
see whether the calendar entries are visible).

I just wonder whether this game is more enjoyable than Heebo ;/

EDIT: I just noticed I have 2 events in December -- but
calendar is showing only one