What is the meaning of the phrase "white-knuckle"?
Hey, Stuart, you got anything going on for Valentine's Day?
Not really.
Other than hiding all the sharp objects around here and white-knuckling it till morning.

Question

Answer

When you clench your fists or grasp something strongly, your knuckles turn white.  This could mean either excitement or tension, and in your example the speaker has changed the expression into a verb to mean that he'll be extremely stressed and nervous the whole day.