I'm sure each store is different but I know someone who used to work at the facility where they printed the WM ad and the security was CRAZY. They were searched at the end of shift and had to sign non-disclosure agreements, etc. If I recall correctly, it was printed in October then but with technology advancements, I'm sure it's not printed until closer to release now. It arrives at the newspaper sealed in black plastic and they agree not to open them until the day before circulation.
When I worked for Kmart, I had the BF ad 4-6 weeks ahead so I could make sure inventory was ordered in time for arrival. I have no idea if that's they way they do it now of if it's all electronic.