Why do they not celebrate birthdays or any religious holidays?

Some groups, such as certain Jehovah's Witnesses and other religious sects, do not celebrate birthdays or religious holidays due to their beliefs that these practices are rooted in pagan traditions or do not align with their interpretation of scripture. They may view such celebrations as distractions from their spiritual life or as forms of idolatry. Additionally, they often emphasize a focus on worship and community over personal milestones or secular festivities.