By an act of worship.
they celebrate their gods
they give us gods grace
They celebrate the gods with sacrifices of animals
Judith
Throughout the Dynasties of Egypt many gods and goddesses were celebrated.
Shinto priests celebrate all of thir gods birthdays
to abaye gods
To celebrate the Greek gods
it was to appease the gods. they made this so they can know what days to celebrate the gods triumph
Hinduism has an ancient, highly sophisticated culture of arts, music and dance which celebrate the gods.
You shouldn't press anyone to celebrate unfamiliar holidays. But if you can celebrate Hanukkah in their presence, and maybe teach them about the history and reasons behind it.
Purim.