在英语中，"这几天很愉快"可以用以下句子表达：

"These past few days have been really enjoyable."
"I've had a great time these past few days."
"It's been a pleasure these past few days."

这些句子的意思都是在描述过去几天的感受，表达了过去几天很愉快的感觉。

关于来源，这些句子都是由英语中常见的动词和形容词组成的。例如，"enjoyable"是形容词，意思是"令人愉快的"；"great"是形容词，意思是"很好的"；"pleasure"是名词，意思是"愉快"。

下面是这些句子的中文翻译：

"These past few days have been really enjoyable."：这几天真的很愉快。
"I've had a great time these past few days."：这几天我玩得很开心。
"It's been a pleasure these past few days."：这几天真是令人愉快。