Usually at the same time for men, though not always. I did both the summer before I went on a mission. For men who serve missions, most do it this way.
I believe all the women in the congregation get together once a week during the week. When I was a kid, these meetings often included craft-making and so forth. This could account for the sudden eruptions of identical decorations. I think they are trying to branch out and do more varied stuff, but I think it probably varies wildly depending on location and the average age of women in the congregation.
This is probably the only thing I really miss about being a Mormon. There’s a ready-made community anywhere you go and there are endless opportunities to meet new friends and socially interact with a wide variety of people. If only it didn’t have to come with a really repressive religion attached…