In the following days (comma)

[Mad-ox]

Do I have to use comma after: The following days

THE FOLLOWING DAYS, he forgot about the discussion.

 

[emsr2d2]

Do I have to use a comma after: The following days

THE FOLLOWING DAYS, he forgot about the discussion.

In your title, you remembered to use the word "In" at the beginning if the phrase but in your actual post, it has vanished. I would probably have said "Over the next few days ...".

There was no need to put the first three words in capitals.

The comma is correctly used.