What does "a do" mean in British English?

A social event - a party, celebration, or gathering, usually of some scale.

"Are you going to the work do on Friday?"
"We're having a bit of a do for her birthday."
"The wedding do was lovely."
"It's a black-tie do." (formal event)

Almost any social gathering - office parties, birthday celebrations, weddings, retirement parties, neighbourhood events. It implies more than a quiet dinner but doesn't have to be huge.

British informal. Americans would say "party," "event," or "function." The word do in this sense is distinctly British and can sound quite charming or old-fashioned depending on context.

dos (the parties) - not do's

A separate meaning - hairdo = hairstyle. The two meanings are unrelated.

• "Party" - universal
• "Gathering" - slightly more formal
• "Bash" - informal British/Australian: "a birthday bash"
• "Shindig" - informal, slightly old-fashioned: "quite a shindig"
• "Function" - corporate events: "the Christmas function"