Yes, BUT you won’t be able to play the event. 

You can join or leave a Den whenever you want; you are not forced to stay in a Den nor will you be blocked from entering a Den just because an event is active. However, please do remember that in order to play the event, you will need to choose your Den before the event starts.
Otherwise, as we mentioned in the “event entry” rules, if you switch dens during the event, you will lose your right to play.