Muslims use an uncorrected lunar calendar, and as a result their holidays slip through the seasons at a rate of about 11 days per year. The reason is not ignorance of astronomy, but a deliberate effort to follow a different schedule from that of any other faith.
This creates an interesting situation during the month of Ramadan, when faithful Muslims are expected to fast and abstain from drinking from sunrise to sunset. When Ramadan falls in mid-winter, this imposes no great hardship, since days are short and cool. Fifteen years later, however, Ramadan falls in mid-summer, when days are long and the heat makes people quite thirsty. That is when many in Arab cities wait impatiently for the boom of the cannon which traditionally announces every evening the end of the fast.