People usually say “Happy Hanukkah” starting shortly before the holiday and throughout all eight days.
During the holiday
It is common and fully appropriate to say “Happy Hanukkah” on any of the eight nights or days of the festival, whether at the beginning or later in the week. You can also offer the greeting when you first see or message someone during Hanukkah, even if candles for that day have not yet been lit.
Before and after
Many people are comfortable receiving Hanukkah greetings for about a week or two before the holiday begins, especially in cards or early well‑wishes. After Hanukkah ends, it is generally better to switch to a neutral greeting (like “hope you had a nice Hanukkah” or simply “have a good day”) rather than saying “Happy Hanukkah” in the present tense.
If you are unsure of dates
Because Hanukkah dates change each year, checking a current Jewish or general calendar helps ensure your greeting falls within the holiday period. When in doubt, a broader phrase like “Happy holidays” or asking “Are you celebrating Hanukkah this week?” lets the other person guide the conversation.