Most panthic scholars have no issues with puratan Singhs coming together to celebrate SIkhi around the same time Diwali occured its only these modern day missionaries such as DHunda who question this practice.
In Ancient times, Hindustan had two main celebrations " Diwali" and Vasakhi Mela". These were not day celebrations instead they were week celebrations. Through out the week people would celebrate by performing hindu rituals. During the time Sri Guru Angad Dev JI , Sri GUru Ji recognized some misinformed local milgobhan Gursikhs were celebrating these Hindu holidays in their respective village according to Hindu Custom. So Sri Guru JI gave the Hukum that during these dates Gursikhs should come together collectively to the Gurus city for GUru sangat and celebrate Sikhi via kirtan for weeks ( smagams) instead of celebrating a Hindu tradition with HIndus.
Through Bhai Daya Singh Jis rehatnama we know Singhs would celebrate diwali and Khalsa Purab ( vasakhi) in Hazoor Sahib. ALso, we know through Guru Khalsa Panth ( Rattan SIngh) that it was custom for Gursikhs to gather in times of Diwali. Rattan Singh mentions how Shaheed Bhai Mani SIngh JI invited SInghs from far and wide to come to AMritsar to celebrate on the day of Diwali. Many people question Sri GUru Hargobind Sahibs release because they say this date was purposely fixed so we celebrate diwali like the Hindus. However, this doesnt make sense because Sangat was already getting together on diwali well before chheva paatshah.
Now the question arises why did Guru Sahib tell Gursikhs to celebrate on these days? THink in practical terms. Most of us who live in the West are well familar with the two main holidays " Christmas" and " Easter". We have seen many milgobhan Gursikhs who live far from sangat celebrate these traditions according to Christian tradition as they are highly influenced by their surroundings. For example, during Christmas they buy a Christmas tree and exchange gifts. But more prudent/sojhivan Gursikhs use this time to join and celebrate Sikhi through kirtan smagams or akhand paaths. No we are no celebrating Christmas or Easter. instead we preventing sangat from straying to other religions/traditions. Such practices are important for Gursikhs living in areas were the majority are not Gursikhs. Likewise Sri GUru JI used these dates as a means for far and widr sangat to keep distinct from outer influence.
Vasakhi ( birth of Khalsa) which in April occurs at the same time as Vasakhi mela which is festival celebrated by Hindus . We are quick to question date of bandi chhor divas but we dont think twice about Vasakhi? Guru Sahib also knew it was important for SIkhs far and wide to meet throughout the year so they can be aware of important information pertaining to panthic affairs. FOr example, on one diwali Sri GUru Raam Das Ji announced to sangat that the next GUru is going to be Sri GUru Arjan Dev JI. Think logically it would make sense to make such annoucements at a time were a majority of the panth came together this way some anti-panthic elements could not try to claim gurgaddi. We also know on the day which Khalsa was born there was a large crowd present and it was during Vasakhi. This way a majority of the sangat was well aware of the mischevious mahants and the new order of the khalsa, otherwise Mahants would have continued misleading Gur-sangat.
Harmeet SIngh Jee, we need to be full aware of modern day Mahants such as Dhunda who are trying to mislead and confuse the sangat.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥