Wording A Casual Drop-In Birthday Invitation

I`m thinking of having a birthday party where it will start at around 4pm in the afternoon. Rather than the organisation of a large, structured party, I`d like to have people drop in, stay for whatever time they like, and leave. That way I don`t have everyone there at once, and it can be quite a casual affair. How do I word this on an invitation, so that people come, but realise it is nice and informal?