外公的生日英文是 grandfather's birthday。外婆的生日英文是 grandmother's birthday。

"Grandfather" 和 "grandmother" 都是英语中表示祖父和祖母的词。这些词都来自拉丁文，"grand-" 意思是 "great" 或 "grand," 而 "-father" 和 "-mother" 分别表示 "father" 和 "mother"。

下面是一些使用 "grandfather's birthday" 和 "grandmother's birthday" 的例句：

"I can't wait to celebrate my grandfather's birthday next week. I'm going to make him a cake and give him a card."
"My grandmother's birthday is coming up soon. I'm thinking of getting her a bouquet of flowers and taking her out to dinner."
"We always have a big party for my grandfather's birthday. All of his children and grandchildren come to celebrate with him."
"I love celebrating my grandmother's birthday. She always makes her famous chocolate cake and we have a dance party in her pving room."

下面是这些句子的中文翻译：

"我迫不及待地想下周庆祝我外公的生日。我会给他做一个蛋糕，送他一张卡片。"
"我外婆的生日快到了。我在考虑给她买一束花，然后带她出去吃饭。"
"我们总是会为我外公的生日举办一场盛大的派对。他的所有儿子和孙子都会来庆祝。"
"我喜欢庆祝我外婆的生日。她总是会做她那著名的巧克力蛋糕，然后我们在她的客厅里跳舞。"