I believe there were 3 Adventures this past update, because there was only one new game released.
The date you finish all Adventures isn't as significant, except when it would be update time. All, or at least most, of the dates are listed on the website as to when the updates are scheduled. February had an update on the 19th, so that's probably why you were done around March 7th. The next update is March 19th, so you'll be able to do the Adventure you're waiting for then.
Don't worry, you aren't doing anything wrong.
Sounds like you've been on top of the game, and are all up-to-date on things. I just finished the last Adventure to get all caught up a few days ago, so I'm like you, waiting for the next update.
~Vicks