It's similar to "Well (then) " and something like an indirect introduction or a segway to a farewell: "Well, then ... it was a nice evening. Bye!"
It's often used sheepishly, when somebody wants to say farewell, but is too embarrassed to say it frankly: "Now then ... it was a nice evening" actually meaning "Please leave now. Good night!".
Or, sometimes, it's the opposite, that is, he doesn't dare to say he actually wants to continue the evening but finds it too obstrusive to say it frankly: "Now then...it was a nice evening..." actually meaning "I want you to stay, but I don't dare to say so."