Guru Amar Das first institutionalized this as one of the special days when all Sikhs would gather to receive the Gurus blessings at Goindwal in 1567. In 1699 Guru Gobind Singh gathered thousands at Anandpur Sahib and founded the Khalsa order by baptizing 5 brave Sikhs who were willing to give their life for the Guru. The Five Beloved Ones in turn baptized Guru Gobind Singh into the Khalsa brotherhood. This day celebrated around April 13 is considered the birthday of the Khalsa order. Sikhs visits gurdwaras and fairs and parades are held. Many Sikhs choose to be baptized into the Khalsa brotherhood on this day, as well the wrappings of the Nishan Sahib flag post at most gurdwaras are changed on Vaisakhi.
Note: Vaisakhi is not the Sikh New Year. Vaisakhi occurs in the second month of the Sikh calendar. The Sikh New Year is on Chet 1 according to the Sikh calendar which occurs on March 14th every year. See the Sikh Calendar for more information.