While culinary historians debate its exact lineage, most agree eggnog originated from the early medieval Britain “posset,” a hot, milky, ale-like drink. ... By the 13th century, monks were known to drink a posset with eggs and figs.

Is traditional eggnog a British or American custom?

Eggnog is a traditional 'American' drink but it started life in the UK as a kind of 'posset' (hot milk mixed with wine or ale and spices).

When did eggnog become a Christmas tradition?

The custom of toasting to the new season with this festive cocktail actually began during Britain's early medieval years, and the drink later became popular in the American colonies by the 19th century. However, per TIME, eggnog has been associated with Christmas since the 1700s.