Mexico's  Ambassador To The United States, the Mexican-Armenian Arturo Sarukhan tweets:

• Arturo_Sarukhan On my way to commemorate #CincodeMayo with Minority Leader Pelosi & other US Representatives on Capitol HIll about 2 hours ago via web

• Arturo_Sarukhan
  Llegando al Capitolio para conmemorar el #CincodeMayo con la congresista Pelosi y otros legisladores estadounidenses about 2 hours ago via web

Right! I'd forgotten about this holiday, which is a Mexican rather than American holiday—but I can see why "congresista" Pelosi would be happy to help celebrate it, since her party gets most of the Mexican votes in the elections.

I still don't understand why Republicans do it.