As we all know on some years there simply is no event in the summer for all to participate in.
So here is my suggestion:
Make an event which can be used in each year where there is no summer event for like Football or Summer Olympics or whatever.
This can still be calendar based but simply doesn't use single event specific resources (so no footballs etc) but gives a reward in items for each day door when a specific goal is reached.
Next is a SUGGESTION SO DON'T BITE MY FREAKING HEAD OF IF IT IS NOT SOMETHING YOU LIKE INSTEAD SUGGEST SOMETHING YOURSELF FOR THESE DOOR -_- (had to do that sorry)
We would have 30 doors.
Each have a specfific task.
These can be personal, these can be guild or server oriented.
When the goal is reached in time (aka a 24H period) the door unlocks.
Behind it is either some resources or maybe a building or whatever.
An example can be: X amount of adventures done within 24H.
Or: Find X amount of X resources by your explorers etc etc.
This event can then be reused every year there is nothing in the period between easter and halloween.
The exact specifics can be worked out by BB or by perhaps getting more ideas from the community.