How to plot power spectrum when sampling period is 1 day and Number of samples are not equal to 2^n ..........?????

The daily Temperature data for four months. Now I would like to analyze Energy contained in which time periods..? Like the energy is in 20 to 30 days or 10 - 20 days or 30 -60 days..

To be precise, if we have Solar Radiation data daily for a year, in poles (North pole). The spectrum would be something like for one day period one peak and seasonal peak it should definitely shows.

Similarly I would like to analyze this data please provide me some suggestions and code also.