Executor event unavailable?

It’s on the 15th of every month, I’m in the same boat as you 

oh, I still need 2 ships at seven stars so not this month for me but probably next month 

Its 1x per month. It's not like the other ship events available all the time. Not sure why CG thinks this is good for the game, but that is what is going on. Once it is available around the 15th, you can do the event.