Minimum temperature of a year is given in GRD file. I have to read that file and then calculate the frost day (less than zero degrees celcius) from it in python.
What are you using to do this?
In brief, you can load the data using RasterIO and then when you have a numpy array, you can do something like
numpy.where( data < 3 ) and sum the resulting fields. That should give you an array that is X by Y and has a single value, which is a count of the number of days that are sub-zero. RasterIO again has a write function.