Panel 1: We find ourselves looking at a downcast Elizabeth staring out the picture window with a look on her face like something is the worst thing ever when she whines about the fact that it's raining again. The reason she's about to thought-bubble the word "GLOOM!" is that it's the morning of Christmas Eve Day and it doesn't look like Christmas at all.
Panel 2: John walks over and asks her what's the matter because she looks sad.
Panel 3: He sets up the mawkish punchline by asking her what he can do to cheer her up.
Panel 4: When asked if he can make it show, he's mildly confused because he doesn't remember that he's living in a sitcom so the desperate look on her face confuses him.
(So would the whining about the poor, sad children in the Southern Hemisphere cruelly cheated out of Christmas MAGIC because nobody has gotta kill themselves shoveling driveways this time of the year.)
Summary: Liz seems to have gotten used to piles of snow on the ground before Christmas and the green Christmas that no one else minds looks like someone up there doesn't like her any more. Also, this is one of the last toddler jokes she gets before next year when she gets all the neglected middle child jokes ever.