1. As per the Solar calendar, every year the months fall in the same respective seasons of that location. For example, the months of March, April and May in India will always have summer, while July, August, September will always have monsoons i.e. rains in western India. November, December and January will always have winter.
2. In the lunar calendar in different years the months fall in a different season.
The lunar calendar has about 11 days less than the solar calendar. Thus, every year, the month of the lunar calendar occurs 11 days prior to what it had occurred in the previous solar year. Thus, in a span of about 33 lunar years a human being will experience all the different seasons for one particular month of the lunar calendar. This is very important because the yearly activities of a Muslim are based on the lunar calendar. Certain months like Ramadhan and Hajj are very important to the Muslims. During Ramadhan a Muslim has to fast which includes abstaining from food and drink from sunrise to sunset. If the Islamic months were based on the solar calendar where the seasons were fixed, then people living in certain parts of the world would have Ramadhan in summer while in other parts of the world it would be winter. Some Muslims would have to fast for a longer period of time where the days are long while other Muslims would have to fast for shorter period of time where the days are short. If the seasons did not change, then Muslims living in some parts of the world may feel that they are at a disadvantage throughout their lives.
By following the lunar calendar, every Muslim has a taste of fasting in different seasons and for a different time period, in a span of about 33 years of his life.