Is there a word for days of mental clarity?

Solution 1:

They have lucid days.

Showing or having the ability to think clearly, especially in intervals between periods of confusion or insanity:

but it's not usually applied to people whose mental processes are normal all the time.

Example:

"We have heard from several people that close relatives with Alzheimer's disease became amazingly lucid for short periods of time after receiving narcotic pain relievers."

More examples:

• When Alzheimer's Patients Make Perfect Sense: Those Stunning Moments of Total Lucidity

• Alzheimer's Symptom: Has sudden lucid moments

• Why Do Alzheimer's Patients Have Lucid Moments?