あけましておめでとうございます。

Akemashite omedetō gozaimasu.

Happy New Year.

Word for word

“Congratulations on the opening (of the year).”

Politeness: Polite

です/ます style. The safe default with strangers, shop staff and colleagues.

When you would say it

From 1 January, for the first few days.

Worth knowing

Do not say it before the new year starts.