Seasonal variation in what? Answering that question is where I would start. After that, you could separate the data by season, or into smaller divisions of time, and you could do a number of analyses on each division.
I think you would start off by defining what you mean by seasonal variation and then you may have to consider a retrospective study. Determine where you want to start from and the cut off point of your study. e. you may want to start from 2000 to may be 2010