Rabi' al-awwal (ربيع الأوّل) is the third month in the Islamic calendar. During this month, majority of the Muslims celebrate Mawlid - the birthday of the Islamic prophet Muhammad. Shi'a Muslims believe the exact date of birth of Muhammad to have been on the seventeenth of this month. The name Rabī‘ al-awwal means the first [month] or beginning of spring, referring to its position in the pre-Islamic Arabian calendar. Hence this is considered to be a very blessed month.
The word "Rabi" means "spring" and Al-awwal means "the first" in Arabic language, so "Rabi' al-awwal" means "the first spring" in Arabic language. The names seems to have to do with the celebration events in the month as "spring" is the end to winter (symbol of sadness) and consequently the start of happiness. The Arabic calendar being lunar calendar, the month is naturally rotating over years and Rabī‘ al-awwal can be in spring or any other season every now and then, so the meaning can not be related to the actual season.
The week including 12th and 17th of is called Islamic Unity Week in Islamic countries to address both Sunni and Shia views on the birth date of Prophet Mohammad.
Although historians and scholars disagree on the exact date of Muhammad's birth, it is commonly celebrated on 12th or 17th of Rabi' al-awwal. The celebration of the Mawlid is done differently depending on the country. In some areas celebrations begin as early as the first of the month and can continue till the end of the month. Muslims generally put coloured lights on roads, streets, and their homes and put green flags as well to celebrate. In many countries a procession is also conducted on 12th or 17th of Rabi' al-awwal night and day. On these occasions sweets and drinks are also distributed widely from home to home and to the general public. In some areas Muslims also exchange gifts.It is the month of blessings.