Wedding venue releasing dates....

It does seem odd/rude for them to have not given you a heads up and asked you if you still wanted the date, but as you hadn't paid a deposit I can understand them selling the date, as you had no contract/agreement.

Personally I didn't think you could hold dates without a deposit - did you have anything signed/in writing saying the date was yours?

While it does seem a little rude to not ask if you still wanted the date, many venues end up with people "penciled in" and never hear from them again. A large number of couples "panic book" and then make their choice later once they've calmed down enough to review their options

I'm my experience they will reserve the date for a short time usually from as little as two weeks to a few months depending on the venue, but after that time is up, the date is on sale again if you have not placed a deposit, due to my above point. As you left it pretty much a year without paying the deposit, the venue has the right to sell the date. Because nothing is "confirmed" until you pay the deposit and sign the contract with the venue.

Or it could be completely innocent that one person had the understanding it was your date, but as a deposit wasn't paid, another person has gone "oh yes this date is available" to another couple.

That situation is really weird but then so is having a date without a deposit. We were allowed to hold our date for a month by which time we had to have paid the deposit but they clearly told us that. Did you never discuss payment schedule or ask about deposit...?