A date
11^11
Easter
piyusha
A date
Today
It is: TODAY
Today
Today (considering you'll read this answer today).
A date or Today
If you write April 26, 2011, this contains only 5 letters, but unfortunately also contains numbers...
One sentence using all the letters in "these letters" could be: "Settle these letters on the shelf for later use."
Even if you used roman numerals, it would still require more than 5 letters.