A few days

A few days typically refers to around 3 days. It is more than a couple days (two) but less than a week (seven). So a few days could range from three to six days but generally is three days.

I’ll pay you back in a few days.

End of the week

If someone says end of the week, they could be referring to Friday, potentially might mean Saturday or it may be Sunday they’re talking about. Either way, the end of the week is sometime around the weekend. That is why it is called the week end. It happens at the end of the week.

I will pay you back by the end of the week.