Sikh anniversaries were kept for three centuries on the Bikrami calendar, a lunisolar system in which dates move against the solar year. Gurpurabs therefore fell on different Gregorian dates each year, and occasionally twice or not at all.
Pal Singh Purewal, a Punjabi Canadian engineer, worked out a solar calendar fixed to the tropical year with the epoch at 1469, the birth of Guru Nanak Dev Ji. Months were named from the Barah Maha in the Guru Granth Sahib Ji: Chet, Vaisakh, Jeth, Harh, Sawan, Bhadon, Assu, Katak, Maghar, Poh, Magh, Phagun. It was adopted by the SGPC in 2003.
It was amended in 2010 in a way that returned several major dates to lunar reckoning, and the amendment was rejected by Takht Sri Damdama Sahib, by Sikh bodies abroad and by a large part of the diaspora. Since then different Sikh institutions have observed different dates for the same events in the same year.
The dispute looks technical. It is about who has authority to settle a question for the whole Panth, which since 1925 has been an elected committee in Amritsar whose electorate is confined to one country.
Guru Nanak Dev Ji's own gurpurab remains on the Katak full moon and moves, which is the one date the reform did not fix.
The calendar adopted in 2003 fixed the Sikh anniversaries against the solar year so that a Gurpurab would fall on the same date each year.
The practical weight of it falls on families abroad, who book leave and halls a year ahead and cannot do it against a date that moves. That is a large part of why the diaspora pressed hardest for the fixed version.
The argument is about authority as much as about astronomy. Who can change the calendar of a community with no central clergy is the question underneath it, and it has not been answered.