WinterNew Years: Here are some of my favorite New Year's related bulletin boards:
Snow and Cold: Something we could technically keep up for quite awhile here in Michigan:
Valentine's Day: You can do the love aspect or my favorite, the candy/treat aspect:
St Patrick's Day: I've cut out shamrocks and and titled the bulletin board "Our Library Rocks".
Bunnies: The most obvious choice of wording is "Hop Into the Library".
Flowers: I've done "April Showers, Bring May Flowers" and "We have Blooming Good Books!"
Eggs: You could have the kids write book titles on eggs as well:
National Frog Month (April): Learned this from Brownielocks but haven't used it yet.
Poetry Month (April): I've done a "Poet-Tree". I remember seeing a blog post that describe making a program out of creating a "Poet-Tree" recently but can't find it. Let me know if this sounds familiar.
Earth Day: You could say "Reduce, Reread, Recycle".
Pet Month (May): It would be very cute to let kids bring in pictures of their pets to display.
I mainly do stuff related to our SRP theme, but have used these for the couple weeks between when Summer Reading ended and school started:
Trees: "Leaf through a good book" is a fall favorite.
Halloween: I've used Jack O' Lanterns and labeled the bulletin board "Carve Out Time to Read" or "Pump Up Your Brain". I've also done ghosts and titled it "We've Got Spook-tacular Books".
Popcorn Popping Month (October): One idea that could definitely be used other times of year as well.
Thanksgiving: Turkeys are always fun. I've also done a food related bulletin board with the phrase "Books are Brain Food".
I hope these ideas are useful. Let me know if you have any to add :)