First, the ad sales staff will play around all month and then in the last few days realise that they have to sell all the advertising space, and on the last day, actually end up giving most of it away at far below rate-card.
Meanwhile, the editor is trying to get commissioned articles, photos, etc from freelancers, who will never submit on time, often weeks late, and when they do it requires extensive rewriting and editing.
Then the subediting process holds everything up until almost the last minute.
Then the art editor can get the blame each month for making the entire mag late - as they're the one at the end of the production process.
Then, when it's finally out the door, we all go down the pub.
That's how a magazine gets printed.