Rabi' al-awwal (ربيع الأوّل) is the third month in the Islamic calendar. During this month, the majority of Muslims celebrate Mawlid - the birthday of the Islamic prophet, Muhammad. Although the exact date is unknown, Sunni Muslims believe the date of birth of Muhammad to have been on the twelfth of this month, whereas Shi'a Muslims believe him to have been born on the dawn of the seventeenth day. 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.
Rabi' al-awwal Wikipedia
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.
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.
The Islamic calendar is a lunar calendar, and months begin when the first crescent of a new moon is sighted. Since the Islamic lunar calendar year is 11 to 12 days shorter than the solar year, Rabī‘ al-Awwal migrates throughout the seasons. The estimated start and end dates for Rabī‘ al-Awwal are as follows (based on the Umm Al-Qura calendar of Saudi Arabia):01 Rabī‘ al-Awwal 897 AH, the fall of the Emirate of Granada, the final Muslim kingdom of al-Andalus
08 Rabī‘ al-Awwal, death of Imam Hassan Al-Askari Twelver Imām, Hasan al-‘Askarī (see: Chup Tazia)
09 Rabī‘ al-Awwal, Eid e shuja
12 Rabī‘ al-Awwal, Sunni Muslims observe Mawlid in commemoration of Muhammad's birthday
17 Rabī‘ al-Awwal, Muslims celebrate the birthday of the Imām, Ja‘far al-Sādiq.
18 Rabī‘ al-Awwal, birth of Umm Kulthum bint Ali
26 Rabī‘ al-Awwal, death of Abu Talib ibn Abdul Muttalib
26 Rabī‘ al-Awwal 1333 AH, death of Khwaja Sirajuddin Naqshbandi, a Naqshbandi Sufi shaykh
Other events:The Hijra (migration) took place in this month
Eid-e-Zahra (a.k.a. Eid e shuja), a celebration of Shi‘ah Muslims
Marriage of Muhammad to Khadijah bint Khuwaylid
Building of the Quba Mosque (first mosque in Islam)
The week including 12th and 17th is called Islamic Unity Week in Iran to address both Sunni and Shia views on the birth date of Prophet Mohammad.