The 15th of Av is known as being the "Day of Love" (יום אהבה). This is not just some modern day marketing title, but this day was already called a festival (Chag) from ancient times as it is written in the book of Judges (Shoftim) 21:19 - "Then they said, Behold, there is a feast of Hashem in Shiloh yearly...."

The Talmud elaborates on this Special day as written  (Ta’anit, Chapter 4)

Rabban Shimon ben Gamliel is quoted saying:

"There were no better (i.e. happier) days for the people of Israel than the Fifteenth of Av and Yom Kippur, since on these days the daughters of Israel/Jerusalem go out dressed in white and dance in the vineyards. What were they saying: Young man, consider whom you choose (to be your wife)? "

Besides the popular reasons why this day was chosen to be a special day of celebration, there remains a reason that is most important and the least known or spoken about! This reason goes all the way back to the time of Moshe in the desert with the Children of Israel, as they were about to enter the Land of Israel.

Here was the original plan!

When the spies came back from spying out the land of Israel, that night should have been a night of great happiness and rejoicing. That night, being the 9th of Av, should have caused the Children of Israel to experience such an intense  feeling of love, that would have endeared us to Hashem completely. The next morning the nation would have begun their march to enter the Holy Land of Israel. On the following day, they would have entered it. The next day, Hashem would have instructed Moshe to have another 3 day preparation period for the Children of Israel to purify themselves, similarly to the 3 day period right before the giving of the 10 commandments. On 15th of Av, Hashem would have brought down His Holy Temple (Bet Hamikdash) from the heavens that was made by His "hands", and betrothed the Children of Israel, under the wedding canopy -"chupa" there, taking them as His "wife" forever! G-d is still waiting for this day!