1) Important birthday, to some 2) Ordinal 
• (a.) Sixth after the tenth; next in order after the fifteenth. • (n.) The quotient of a unit divided by sixteen; one of sixteen equal parts of one whole. • (a.) Constituting or being one of sixteen equal parts into which anything is divided. • (n.) The next in order after the fifteenth; the sixth after the tenth. • (n.) A...
Six'teenth` adjective [ From Sixteen : confer Anglo-Saxon sixteó...a .] 1. Sixth after the tenth; next in order after the fifteenth. 2. Constituting or being one of sixteen equal parts into which anything is divided. Sixteenth note (Mus.) , 
In music a sixteenth is an interval comprising two octaves and a second.
[adj] - coming next after the fifteenth in position 2. [n] - position 16 is a countable series of things
16th adjective coming next after the fifteenth in position