What groups don't observe a December/winter holiday?

The ancient Celtic pagans didn’t have one until they borrowed Yule from the Norse pagans. They celebrated Samhain around November 1st, then went into their huts and didn’t observe any holiday until Imbolc around February 1st.

Of course, Jehovah’s Witnesses do not observe any holidays whatsoever, believing they are worldly and evil.

Any others?

nm

Muslims don’t observe a December holiday except accidentally. They have a lunar calendar and their holidays shift every year. Zoroastrians don’t have a December holiday. Hindus are a group that has festivals about once a week, so some of them fall in December, but none of its ‘major’ festivals fall in December.

December holidays aren’t really all that common. Romans had them, but that’s because Romans had holidays pretty much constantly. Ovid’s Fasti has listed about 5 major festivals a month, so the Romans had a ton. For December alone, they had the Kalends of Neptune, Agonalia, Saturnalia, Opalia, Larentalia, Eponalia, Brumalia, Compitalia, Divalia and at least a dozen more minor festivals. The Ancient Athenians had no major December festivals and most Greek holidays were in summer.

Jehova’s Witnesses, who as far as I can tell don’t celebrate holidays at all.

Probably plenty of groups in areas of the world that don’t have a pronounced winter season/variation in day length.

But even where the day/night variation is less pronounced, lots of groups still have a December-solstice-related holiday. The Hindu holiday Makara Sankranti is a post-solstice celebration, marking the beginning of the solar month after the solstice, so it usually falls in January. It’s not as widely known among non-Hindus as Diwali or Holi but it’s still pretty important, and apparently very ancient.

The ancient Greeks had the Haloea or Festival of Poseidon at the winter solstice, which in fact helped inspire the Roman Saturnalia celebration.

AFAIK pretty much any culture that has a seasonally-based calendar and an agricultural heritage has some kind of celebration linked to the shortest-day solstice. This is true even for cultures which also have a religious calendar that’s not seasonally-based, such as the Muslim calendar. Iranians, for example, who are overwhelmingly Muslim, celebrate the solstice holiday Shab-e Yalda. Ancient Egyptians celebrated the births of the deities Osiris, Horus, etc., during their calendar’s “epagomenal days” after the winter solstice.

Off the top of my head I don’t know of any longstanding cultural tradition in the northern hemisphere that doesn’t have any annual celebration at all related to the winter solstice. (In the southern hemisphere, of course, the December solstice falls in summer.) However, I agree that there are comparatively few such holidays that are widely recognized in modern popular culture.

This not true at all, assuming by “holiday” you mean “celebration” or “festival.” Ancient Zoroastrians had seasonal festivals called Gahanbar that included celebrations of the solstices, Persians, Zoroastrians, and many modern day Iranians celebrate Deygan/Yalda on the night of winter solstice, plus there’s Sadeh on January 30, so technically not December but still in mid-winter. Basically, Zoroastrians use any excuse to get together and have a feast, some of which necessarialy falls during the winter months. Also Zoroastrians in Western countries celebrate Christmas in varying degrees, and of course the New Year, along with everybody else.