(Strip Number 4012, Original Publication Date, 26 May 2010)
Panel 1: We have another single-panel wonder today; going from left to right, we see the end result of Lizzie's party:
1) We start off with the stack of plates presumably ready to be washed; this is because Elly is so in love with doing things the 'right' way, she was foolhardy enough to forego paper plates and disposable cutlery.
2) The "Happy Birthday Banner", table and chairs are strewn with confetti, balloons and other light items that it would take ten minutes at the most to dispose of.
3) Elly sitting on a chair with her face frozen in the Bug-Eyed Glare of Existential Horror™
The reason for this is that not only does she have to clean this up, Mike's happily telling Lizzie "Just think, 'Lizabeth; you get to do this every year!!" reminds her that she'll spend every year being overwhelmed by the horrible burden of spending an hour in the company of small children hopped up on butter cream frosting and Kool-Aid.
Summary: That's because it's not about Lizzie having a nice afternoon; it's about how haaaaaaaaard it is to be Elly Patterson. That's why the next time we see Liz celebrating her birthday, she's sixteen years old and standing around like an idiot going that extra mile to not realize that Anthony would kill his own granny in order to rifle through her trash.