一、一般说“在下午”的时候,要用in the afternoon
He's arriving in the afternoon.
他将于下午到达。
《柯林斯英汉双解大词典》
但是要说具体某一天的下午,要用on the afternoon,后面通常会有of
例句1:I was at home on the afternoon of last Monday.
上个星期一的下午我在家。
例句2:Where were you on the afternoon of May 21?
5月21日下午你在哪里?
《牛津词典》
二、morning和evening也是一样的用法
1.in the morning:在早上
I can't get out of the habit of waking at six in the morning.
我早上六点钟醒的习惯改不了。
《牛津词典》
2.on the morning of:在具体某一天的早上
We got the news on the morning of the wedding.
我们在举行婚礼的那天上午得到这个消息。
《牛津词典》
3.in the evening:在晚上
I usually watch TV in the evening.
我通常晚上看电视。
4.on the evening of:在具体某一天的晚上
On the evening of my 14th birthday,I was given this book.
14岁生日的那天晚上我收到了那本书。