The worst day I can think of would involve too much narrative for a single illustration to cover everything, since it involves a culmination of multiple events stacked together through out an entire day, so I got to thinking about single defining moments that were great or terrible. The one that struck me as being the most interesting for a concept is the day my brother was born. I love the kid, and from an adult perspective, the day he was born was a day I'm so grateful for that I'd count it amongst the best, but I'm not certain I felt that way at the time. What's more, I'm sure that elder children everywhere can relate to just such a sentiment, making it a concept that has the potential to communicate emotionally with a wide audience.
In the sketch I've done, I tried to make the little girl the focal point of the piece, and to highlight the relationship between her and the screaming newborn, by making their faces the only two you can completely see.
I think I need to move the newborn lower in the image, and work out some anatomical issues with the nurses arms, but otherwise I like the direction the sketch is going.