Before 2003, Sikh festival dates were set by the Bikrami calendar, the lunisolar system shared across much of North India, which meant gurpurabs and other observances moved against the Gregorian calendar from year to year in the same way Diwali or Eid do. The Nanakshahi calendar was built to stop that drift for Sikh purposes specifically, fixing each month to a constant solar length so that, in principle, a festival such as Vaisakhi or a Guru's birth anniversary falls on the same Gregorian date every year rather than shifting with the moon.
The month names were kept from the Bikrami tradition, Chet, Vaisakh, Jeth and onward through Phaggan, so the change is one of mechanics rather than of vocabulary, and the calendar's epoch, year one, is set at 1469 CE, the birth year of Guru Nanak, giving the era its name. Vaisakhi anchors the whole structure: it is both the first day of the first month, Chet, and the historic anniversary of the founding of the Khalsa by Guru Gobind Singh in 1699, so the calendar's new year and one of Sikhism's most significant historical events fall on the same fixed date by design.