C2 Continuous curve fit start to end

I have time periodic data that I want to fit a curve to. I am looking for how to fit a curve to the data such that the start and end of the curve fit is C2 continuous as the function wraps around back to the start, when provided with exactly one period of data. The data does not follow a natually continuous funciton (e.g. sine/cosine) that is inherently continuous. Ideally I would like to use the cubic spline fit but I can't quite figure out how to get the C2 condition to work out.

thanks

Greg