There are some studies showing that HRV can indicate best timing for the next hard workout. You might give that a look. The Elite HRV phone app works well.

Here's a study done on a training block: https://www.firstbeat.com/wp-content...m-Hormones.pdf

IME everyone posting here is correct. It's just which part of the elephant one is touching. While it's true that fitness increases best when done consistently all week and week after week, within that structure there are better and worse days on which to do your intervals.