Ramadan is the ninth month of the Islamic lunar calendar, in which fasting was prescribed:

“O you who believe! Fasting has been prescribed for you as it was prescribed for those before you, so that you may attain Taqwah.” - Qur’an 2:183

It is also the month in which the Qur’an was revealed:

“Ramadan is the month in which the Qur’an was revealed…”- 2:185