"Good afternoon" is how you say "中午好" in English. This phrase is often used as a greeting between 12:00 pm and 3:00 pm, when it's considered the middle of the day. If you want to be more specific, you can say "good afternoon" at around 12:45 pm, which is about half-past twelve. Another option is to use "afternoon" alone, as in "Good afternoon! How can I help you?"
