The marriage of the Holy Prophet (SAW) to Bibi Khadija (SA) is reported to have taken place on either 10th of Rabil Awwal or 18th of Shawwal when the Holy Prophet (SAW) was 25 years old.
Excerpt from 'A Restatement of the History of Islam and Muslims':
Abu Talib read the khutba (sermon) of the marriage of Muhammad and Khadija, and his speech proves beyond any doubt that he was a monotheist. He began the speech in the 'Muslim' style by offering thanks and praise to Allah for His mercy and for His countless gifts and blessings; and he concluded by invoking His mercy and blessings upon the newly-weds.
The marriage of Muhammad and Khadija was most successful. It was blessed with felicity unlimited for both husband and wife. Khadija dedicated her life to the service of her husband and of Islam. She spent all her vast wealth in strengthening Islam, and on the welfare of the Muslims.
Khadija had the same sense of mission as Muhammad had, and she was just as eageras he was to see Islam triumph over paganism. To her eagerness to see the triumph of Islam, she added commitment and power.
She freed her husband from the necessity of making a living, and thus enabled him to devote all his time to reflection and contemplation in preparation for the great work which lay ahead of him. This is a most significant contribution she made to the work of her husband as messenger of God. She was the fulcrum that he needed in all the years of preparation for the prophethood.
The marriage of Muhammad and Khadija was also blessed by the birth of their daughter, Fatima Zahra. Though the gifts which God bestowed upon them, were many, there was none that they treasured more than their daughter, Fatima Zahra. She was the “light of the eyes” of her father, and the future 'Lady of Heaven.' The father and mother lavished their love on her, and she brought hope and happiness and the mercy and blessings of God with her into their home.