Vaisakhi is the first day of the month of Vaisakh, in the middle of April. It is the wheat harvest, and it was the harvest festival long before it was anything else.
On Vaisakhi 1699 at Anandpur, Guru Gobind Singh Ji created the Khalsa. That is why it is the largest day in the Sikh calendar and why Anandpur and Talwandi Sabo fill.
On Vaisakhi 1748 the Dal Khalsa was organised at Amritsar. On Vaisakhi 1801 Ranjit Singh Ji was crowned. On Vaisakhi 1919 the crowd gathered at Jallianwala Bagh.
As a harvest day it carries fairs, wrestling, cattle markets and dance. As a Sikh day it carries amrit sanchar ceremonies and nagar kirtans.
It is also new year in several regional calendars across the subcontinent on the same date, which is why Vaisakhi, Bihu, Puthandu and Vishu fall together. The date is astronomical, not doctrinal.
The five who came forward in 1699 were from Lahore, Hastinapur, Dwarka, Jagannath Puri and Bidar, and from castes that would not ordinarily have eaten together. The composition of the five is the argument the ceremony makes.
The wheat is cut in Vaisakh and the fairs follow the harvest, which is why the date carries a harvest and a founding at once.
Jallianwala Bagh happened on Vaisakhi in 1919 because the town was full of people who had come in for the fair.