Boss sends email on Friday 9PM asking for info by Saturday. Calls me on Sunday and blast over phone for not responding to the email even after I told him that I was travelling on some personal work

One thing led to another, he started yelling and I snapped. Now he wants me to apologise to him and has threatened to take HR action. What should I do? It doesn’t feel right to apologise for not looking at email on weekends and being shouted on for this.