Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Setting a specific time to celebrate the birthday of a Wali (pious person) or any ordinary Muslim person and gathering for this purpose in the mosque is a Bid'ah (innovation).
A Muslim should avoid such a Bid'ah. However, it is permissible to make Dua (supplication) for him or give alms on his behalf or the like without gathering for such a purpose. What is mentioned in the question is not considered as Shirk or Kufr but it is an innovation.
We advise you to call them to leave such a Bid'ah and to stick to Sunnah, as the best guidance is the guidance of the Prophet (Sallallahu Alaihi wa Sallam) and his companions, may Allah be pleased with them, who never used to perform or do such acts.
Allah knows best.