January 21.
1813. The pineapple si introduced to Hawaii. While not of tremendous historical significance, I was surprised it wasn't native, so I'll throw this in.
1793. Russia and Prussia sign a partition treaty. Poland ceases to exist as a country until 1815.
1861. As a result of the recent secession of their home states, Jefferson Davis and a few other United States Senators resign their Senate seats.
1968. The Battle of Khe Sanh, Vietnam, begins. Referenced yesterday in Obama's inaugural speech, I will give a brief synoposis of the battle and the even more interesting "Riddle of Khe Sanh" later if I have time.