Hello Sui Generis!
Quite similar case happened to one friend here of Spanish batch. She recieved the flight ticket and visa, when she even hadn't recieved the contract, neither had she confirmed to agree traveling on that date.
So you should take it as a mistake. I mean, that day of Journey will probably be the real one, take it as you will be flying on that date, but they just didn't organize well to send you first the offer of employment and so on... This may have happened cause sometimes it is not your recruiter who arranges your ticket and visa, but this is done by another officer... Yes, it's quite a kaos...
And if you have recieved the email today, note they are on Eid Hollydays, so it might have been like a quick advise to you for getting ready to travel on that date. Thats good, in order to plan everything, isn't it?
By the way, I'm supposed to start training on 14th Dec, so in case you're a girl, we will be in the same batch
Good luck!