Answer:
"On" his birthday
Explanation:
Moving to shorter, more specific periods of time, we use ON to talk about particular days, dates, and holidays . You may hear, “I went to work on Monday,” or “Let's have a picnic on Memorial Day.” For the most specific times, and for holidays without the word “day,” we use AT. We use IN to refer to a general, longer period of time, such as months, years, decades, or centuries. For example, we say “in April,” “in 2015” or “in the 21st century.”
