Quite simple, really. Before, you weren't on too many bulk mailing lists. Somehow, your email address was "released into the wild". There are many ways this could happen, but even if you do everything right, once enough spammers have your email address, the spam can grow exponentially. Just opening an email with HTML viewing enabled can signal the sender that he's got a "live one", making your email address even more valuable for resale to more spammers. As for the date, nearly everything in the header of an email can easily be forged by the sender. I can only guess that the sender puts a future date so that if the email stays in your inbox for a while, it will always be shown first, at the top, getting the most attention.
Last, there are several ways to reduce spam and while you didn't ask for any advice, there's one simple and under-utilized way to nearly eliminate spam from cluttering everyone's inbox. This may be inconvenient for some, but it depends on your point of view about email in general. If you think of it as something purely for your benefit on your terms only, then do this; set up a white list of people and companies you deal with and accept email only from those senders. With all other emails, set your email program to delete them. Don't even send them to your spam folder. If possible, do this at the server level through your ISP. This does get a little hard to manage if you buy stuff or pay bills on line, but still possible to manage. What if someone wants to legitimately contact you, maybe a friend gave another friend your email address, etc.? I say, tough. If you accept this way of thinking, then there's no such thing as a legitimate way to contact me by email unless you have my previous consent and have asked me through some other means to be added to my address book.
Oh, and you'll still get a little bit of spam, when a spammer forges an email seemingly from one of your friends or even from yourself, or when one of your friends gets hacked, but this doesn't happen very often.