Great question, and I'm interested to see what everyone else has to say, but I'll start by sharing my thoughts on it.

I think this tends to be a matter of opinion. Do you watch the Walking Dead? They like to start in medias res (right in the middle of the action) and then go back and tell the history, the story that led the characters to that point in time. It's a great device. However, the first episode of the new season started in medias res, went back and told a little history, then went back to the future, then back to the past again. It had so much back and forth going on that I was completely lost for half the episode and it was making me so mad! I had no idea what was going on. They did it on purpose, for dramatic effect, and someone obviously thought it was a good idea, but the utter lack of background just frustrated me.

On the other hand, you don't want each character to give a biological soliloquy, because that's boring.

I suggest analyzing how much background your reader really needs to know, and when they need to know it.

First, they don't necessarily need to know everything you know. You just need to know all the background because it helps you make good choices, so that your characters act according to their character, with consistency.

But if the reader does need to know some background, try to sprinkle it in as minimally as possible, with just enough information to prevent complete confusion. (A little bit of confusion can be a good thing, especially for a suspense novel or if you're employing a red herring or planning a big reveal later.)

One literary device is called the "As you know, Bob..." It's cheesy if you're not careful, but it can be done successfully. Say two characters, Bob and Sally, are talking about something they already know, but the reader doesn't know. Sally might say something like, "Since the manufacturing plant has been cranking out bad widgets, my husband has been really anxious at home." Now you know about the bad widgets, which is something both Bob and Sally already knew, but Sally wasn't specifically telling Bob about the widgets; she was telling him about her husband's anxiety.

You can also sprinkle background into the narrative:

"Joe's not home for dinner yet," said Sally. Joe had been working late every evening for months.

Just remember that the reader doesn't need to know everything at once. You can wait and reveal more background data in a later paragraph or even chapter. I'd probably like to know why Joe had been working late, and possibly what excuses he's giving Sally, especially if this is a romance novel or drama with an extramarital affair, or if it's a murder mystery, but I can reveal more of that in time. Dropping background hints throughout the book can be a good strategy anyway because it gives the reader "aha!" moments: "So THAT'S why Joe bribed the EPA auditor!"

If you need more background than you can achieve by sprinkling it into the narrative and dialogue, you can consider a prologue or flashback. I've heard mixed opinions about prologues, and some people are really passionate about the fact that you should never use them, but I've ready plenty of them in my life and never stopped reading a book because it had a prologue. Most were written by reputable authors.

However, with both the prologue and the flashback, be careful to draft them as a story, with action and dialogue, rather than a data dump. The background stories you're writing this month are good examples - they force you to think about what happened in the past and use your storytelling skills to explain it rather than listing that first this happened, then that happened, and finally this other thing happened.

My two cents.

Cheers,
Michelle