You can safely say “Merry Christmas” any time in mid‑to‑late December, especially in the few days before December 25 and on Christmas Eve and Christmas Day themselves.

Best times to say it

  • From about mid‑December onward, most people are used to hearing “Merry Christmas” in shops, at work, and in casual chats.
  • The most natural moments are:
    • When you say goodbye in a conversation near the holiday.
    • In messages or emails sent on December 24–25.
  • After Christmas Day, it sounds more natural to switch to “Hope you had a Merry Christmas” and start focusing on New Year wishes.

When to say something else

  • Earlier in December (or from late November), “Happy holidays” or “Season’s greetings” feels broader and fits all end‑of‑year celebrations.
  • In diverse or professional settings where you are unsure of someone’s traditions, “Happy holidays” is a polite, inclusive choice.

Social and online context

  • On social media and in public posts, many people start using “Merry Christmas” in the second half of December as timelines fill with festive content.
  • Trend‑wise, there is an ongoing mix: some emphasize explicitly saying Merry Christmas, while others lean toward inclusive phrases like “Happy holidays,” especially in the US and online communities.

TL;DR:

  • Mid‑December to December 25: “Merry Christmas” is normal.
  • Before that or if you are unsure: “Happy holidays” works well.

Information gathered from public forums or data available on the internet and portrayed here.