Buddha's birthday 2018 in U.S.A. Sunday, April 8

Buddha's Birthday is a holiday traditionally celebrated in most of East Asia to commemorate the birth of the Prince Siddhartha Gautama, later the Gautama Buddha and founder of Buddhism.

It is also celebrated in South and Southeast Asia as “Vesak” which also acknowledges the enlightenment and death of the Buddha.
According to the Theravada Tripitaka scriptures Gautama was born in Lumbini in modern-day Nepal, in the year 623 B.C., according to the Tibetan Account, and raised in Kapilavastu.

At the age of thirty five, he attained enlightenment (nirvana) underneath a Bodhi tree at Bodhgaya (modern day India). He delivered his first sermon at Sarnath, India.
At the age of eighty, He died at Kushinagar, India.

The exact date of Buddha's Birthday is based on the Asian lunisolar calendars.
The date for the celebration of Buddha's Birthday varies from year to year in the Western Gregorian calendar, but usually falls in April or May.
In leap years it may be celebrated in June.

https://en.wikipedia.org/wiki/Buddha%27s_Birthday