Is celebrating non-Islamic festivals (like birthdays) with colleagues permissible?

This is another question where scholars are divided, and the answer differs somewhat between birthdays specifically and other non-Islamic religious festivals. On birthdays: some scholars — particularly in the stricter Salafi-leaning tradition — consider organized birthday celebration an innovation (bid'ah) since it wasn't practiced by the Prophet ﷺ or the earliest generations, and discourage it on that basis, even for children. Other scholars, including a number associated with SeekersGuidance and mainstream Hanafi/Shafi'i fiqh, hold that a birthday celebration is a permissible cultural custom (adah) rather than a religious ritual, as long as it doesn't involve haram elements (alcohol, immodest mixed parties, extravagant wasteful spending) — it's simply marking time passing, not worship.

For genuinely religious non-Islamic festivals (Diwali, Christmas, Holi, and similar, which are especially relevant for India/South Asia office culture), the scholarly position is more consistently cautious: most scholars distinguish between polite, respectful workplace participation that avoids religious ritual elements (attending a colleague's office lunch, exchanging pleasantries, accepting food that is otherwise halal) and actively participating in or wishing someone well specifically for the religious/worship content of the festival, which more scholars advise against. A reasonable and widely-recommended approach for office settings: join in the social/collegial parts (a shared meal, a team gathering) while avoiding specifically religious rituals or greetings tied to another faith's worship.

Source: Islamic Association of Raleigh, "Is celebrating birthdays considered haram or disliked (makrūh) in Islam?"; Islam Question & Answer, "Celebrations in Islam: What is Permissible and What Is Not?"; this remains a topic with real difference of scholarly opinion, particularly on children's birthdays.