Rabi' al-awwal (ربيع الأوّل) is the third month in the Islamic calendar. During this month, Muslims around the world celebrate Mawlid - the birthday of the Islamic prophet Muhammad. Sunni Muslims believe the exact 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.

Source ; http://en.wikipedia.org/wiki/Rabi%27_al-awwal 

Is the third month of the Islamic calendar
The Meaning of the Word "Rabi-ul-Awwal":- The first month of spring.
Auspicious Occasions:-
The Holy Prophet  was born in this month as mercy for all and departed from this world 63 years later
 
Events of  Rabi-ul-Awwal
* The Holy Prophet   migrated from Makkah to Madinah Munawwarah.
* Battle of Buwaat took place in the year 2 A.ll.
*Battle of Saf'vaan took place in the year 2 A.H.
*Abu Bakr's(R. A~) daughter, Asma (R.A.) was born 6 years before Prophecy.
* Ruqayyah (R. A.) the daughter of the beloved Prophet  was born 7 years before Prophecy.
*Abu Bakr (RA.) was appointed Caliph.
Matrimonies within Rabi-Ul-Awwal:-
 
Hadhrat Umme Kuithum was married in this month to the third Caliph; Uthinan (R.A.) in 3 A.H.
 
Deaths in Rabi-Ul-Awwal:-
Mother of the believers, wife of the Holy Prophet  Zainab (RA.) passed away in the year 4 A.H.
Ibrahim (RA.) the son of Maaria Qibtiya & of the Holy Prophet  passed away in the year 1O A.H.
Source ; http://www.inter-islam.org/Miscellaneous/months.htm#RABI-UL-AWWAL