(The full transcription of the lecture is here: https://bilvavi.net/english/droshos-what-do-when-you-are-down; includes a Q&A at the end & specific applications for women.)
In short, "Days of Love" are days when you feel "up," and things are going well, both spiritually and emotionally.
"Days of Hate" are the opposite; they're the "down" times.
Here's an excerpt of a transcription of Rav Schwartz's lecture on the subject (stylistic changes made for added clarity for onscreen reading):
Failures Are Not Failures
When a person is in a difficult time and he doesn’t feel a drive to grow, there are two attitudes he can have.
One attitude is to completely give up:
“That’s it. I’ve had enough, and I can’t take it anymore. Today is a bad day, and I’m only going downhill. I can’t work on myself today.”
But the other attitude he can have is to have a different perspective on the situation:
“My soul needs a break sometimes. I can’t always be growing. It’s really not a bad day – it’s a day that I have to go through, because it’s normal for my soul to feel closed up sometimes, [since Rabbeinu Tam has written that] everyone has these 'Days of Hate'.”
We can compare this to going to sleep.
Although sleep is not productive, is sleep a bad thing?
No one looks at sleep as unproductive, because we all realize that we need to go to sleep in order to reenergize. We can’t function unless we get our sleep.
Taking a break is part of life, and we must not look down at ourselves for this.
The difficult times in our life are thus really not “difficult” or “bad.”
Rather, they are like our sleep.
It is necessary for us sometimes to pull back from growth, and let us not beat ourselves up that something is wrong with us that we don’t feel any willpower to grow spiritually.
If you have those days in which you don’t feel like learning or davening or making any spiritual progress, realize that it’s normal, and that it is actually a necessary part of your growth to go through these times in which you lack motivation.
This can change our whole attitude about life!
Our soul needs to rest sometimes – we can’t always grow and reach higher levels.
This is not a fall from our level – it is a necessary part of our growth.
“Failures” are thus not really “failures” – they are needed for our growth.