They are arranged in decreasing order of frequency. "On and off" is a slight bit more often than "now and again", but they're roughly synonymous. "Every so often and "from time to time" are also roughly synonymous and happen somewhat less often than the first two. "Every once in a while" means that quite some time passes between your visits. The last one means you very, very rarely go to the library -- a literal blue moon occurs very rarely under extreme atmospheric conditions -- most people never see one. You might as well not have a library card at all. ;)

I think that they all roughly mean the same, except once in a blue moon which means very rarely, almost never