Since 1999 Vaisakhi has had a nearly fixed Gregorian date, and the change that produced it caused one of the sharpest internal disputes in modern Sikhism.
Sikh festivals were historically calculated by the Bikrami calendar, a lunisolar system shared with much of north India. Under it, the anniversaries of the Gurus moved considerably against the Gregorian calendar, and some fell twice in a year or not at all.
The Nanakshahi calendar, devised by Pal Singh Purewal and adopted by the Shiromani Gurdwara Parbandhak Committee in 1999 for the tercentenary of the Khalsa, replaced this with a solar calendar of fixed month lengths, its year one set at 1469, Nanak's birth. Under it Vaisakhi is 14 April every year and most Guru anniversaries have fixed dates.
The objection was that this severed Sikh timekeeping from the calendar the Gurus themselves used and that is embedded in the scripture's own references, and that it was a modernising imposition. Traditionalist bodies rejected it. In 2003 amendments were made, and in 2010 the SGPC adopted a further revision that returned several major dates to Bikrami calculation, including, notably, Guru Nanak's own anniversary, which remains lunar and moves.
The result is that different Sikh institutions and different countries print different calendars, and the dispute is unresolved. Purewal continued to argue for the original version until his death.
It is an unusually clear example of a religious community deciding, in public and recently, how its time is to be counted.