Our library trips are THE BEST DAY EVERRRRR! because several years ago, my then-6-year-old wrote a note that said, "TODAY IS THE BEST DAY EVERRRR!" When I asked what made the day so great, he said it was because we were going to the library.
+1 vote for SATURDAY NEW BOOK DAY AND PANCAKES! Even the grownups in our house would be excited about that!
We also have the same thing for weekdays and months. Makes it easy for kids to learn the correct names (and order) when you have names like Tiger Tuesday. Maybe you could expand to teach them read the clock, too.