Christmas Plans
Christmas Plans
[Cueball is standing behind a friend, who is sitting at a computer.] Cueball: Hey, will you be in town the day after Christmas? Friend: Couldn't say— Friend: I'm Jewish.
Cueball: But... how does being Jewish keep you from knowing your plans? Friend: I know my plans— Friend: I just don't know when Christmas is.
Cueball: Really? Why not look it up? Friend: Well, I'm also a physicist.
Cueball: So? Friend: I believe that since I don't observe Christmas, it can't have a definite date.

This comic centers around a joke about Quantum superposition in physics - if you don't observe something, it has all possible states, not a specific one. It is a double-entendre with the word observe meaning both "look at" (physics sense) and "celebrate" (a holiday). One of the most famous examples on this is the Schrödinger's cat paradox.
In this comic, a Jewish physicist does not know when Christmas is. Being a physicist, he believes that since he doesn't observe Christmas, it therefore has no definite date.
Adding to the joke, the physicist's observation is actually valid for certain other holidays (such as Easter) that don't have a definite (i.e. annual) date. Because Easter's date seemingly bounces around at random, it could be said to exist in a superposition of all possible Easter dates, and as he doesn't observe (celebrate) Easter, the physicist would be unlikely to check which date it falls this year. Thus, he would be unsure of when the holiday is celebrated.
The title text refers to another principle in physics where the act of measuring something must also change it in some way.  If one drops a thermometer into a mug of water, energy spent (or released) when heating (or cooling) the mercury in the thermometer changes the temperature of the water in the mug by a small amount.  The only way not to interfere with the temperature of the water in the mug is not to measure it.
[Cueball is standing behind a friend, who is sitting at a computer.] Cueball: Hey, will you be in town the day after Christmas? Friend: Couldn't say— Friend: I'm Jewish.
Cueball: But... how does being Jewish keep you from knowing your plans? Friend: I know my plans— Friend: I just don't know when Christmas is.
Cueball: Really? Why not look it up? Friend: Well, I'm also a physicist.
Cueball: So? Friend: I believe that since I don't observe Christmas, it can't have a definite date.
