Notice that the given function corresponds to a graph of a sine function with amplitude 4 and mean value 2:
Notice that the temperature is above 0ºC since one month before January until 7 months after January. This means that the temperature is above 0ºC from December until August.
This means that the temperature is above 0ºC all January, all July and the months between them.
Therefore, the answer is:
January and July.