protego shield charm

[ktgiii]

No: HPFGUIDX 70188

Hello all.  After almost two years, I'm finally de-lurking.

Harry uses the protego shield charm against Snape during his 
occlumency lessons (ch. 24 p.591 U.S.), and then again during the 
final fight (ch 35 p. 783 U.S.)

Where and when did he learn this spell?  He uses it well both times.  
Obviously, if he knew this in book 4 he would have used it in the 
graveyard. (at least to block the crucio?)

I've read the book only once, and then "chunks" of it multiple times, 
but I just can't find any reference to the protego spell.

It just seems to me that it's an important spell. (And useful, too!)  
If he learned it away from our prying eyes it would be a bit odd.

Thanks for any input!  

-Kathryn