I have this formula in a cell =IF(K2="","","Check Number"&" "&J2&"
"&"in the amount of "&I2&" was mailed on "&(K2)), and on cell K2 i
have the date format as mm/dd/yy, but the result returned this "Check
Number 12345 in the amount of 258.93 was mailed on 39797." How can I
change the 39797 to show 12/15/08?

Instead of just K2, include this as the last part of the formula:
TEXT(K2,"mm/dd/yy")