March 3rd is Girl's Day, or officially Hina Matsuri.  Matsuri means festival.  On this day Hina dolls are dressed up and displayed in houses, department stores, temples, etc.  Any household with a daughter, drags the dolls out of the closet and puts up a doll display to celebrate Girl's Day (much like our Christmas tree custom).  This is the display Yuki's Mom put up.  There are also traditional snacks for Girl's Day and some people dress the girls up in a kimono and parade them to shrines.  (I was too busy to get out and get pictures of that).

May 5th is Boy's Day but it is now called "Children's Day"; I figure the name change was because May 5th is a national holiday and Girl's Day isn't so rather than making another national holiday on March 3rd to make things fair, the government changed the name of Boy's Day to be accommodating of both sexes (just my theory).  Note the pattern of the holidays: March 3rd (03/03), May 5th (05/05). 

The reason for no holiday on April 4th (04/04) is that for the Japanese the number 4 is like 13 to us.  Why 4?  It is because four is pronounced shi and that also means death in Japanese.