In Back to the Future Part II, it was mentioned that Biff turned 18 in 1958. That's when he placed a bet on races and won his first million. So why did the old Biff give the younger himself the almanac in 1955, not in 1958?
I think the other answer dances around this but...
Think back on your life. When would you love to go back and talk to yourself at some prior point? (not necessarily to give yourself a future sports almanac) My bet is you'd go back and pick the lowest point in your life.
This was the night life changed for Biff. Before that, he was the top dog. He was strong and feared. He ran a gang. He could do as he please. Then came that night. The night George McFly laid him out with a single punch. Biff would spend the rest of his life in George's shadow. Not only did old Biff's plan work, it gave him revenge on his nemesis, George.
Another speculative answer on my part, here goes.
Ok, so you are a crochety old man standing on a street corner and overhear someone talking about a time machine and taking back a sports almanac to use to get rich. You decide that you are going to steal the machine and give the book to yourself in the past. You have a very short window of time to accomplish this with no prior planning. Where do you go? What date in time do you travel to? Just how many of us are going to remember an exact place and time that we can meet ourselves 60 years earlier? You want to make sure you hand deliver the book to yourself so going back and mailing it or leaving it in your mailbox won't cut it.
The Enchantment under the sea dance was obviously a memorable event for those kids as George and Lorraine seem to talk about it all the time. And the actual date of the dance could have been easy to remember by a 74 year old man. Additionally, Biff knew he would be at that dance which means he could find himself at home before it started that evening. And lastly, at what better time in your life to give yourself a book that will make you rich then when you are in high school with your whole life ahead of you?
This is why I think he chose to go back to November 12, 1955. He knew an exact date, time and place where his younger self would be.
Actually, with gambling being illegal until the age of 21, Biff actually gives his 17/18 year old self (depending on whether his birthday has already passed that year) the almanac in 1955. The film states he made his first million in 1958 on his 21st birthday, when he was legally able to gamble.