Yes, you do.
Technically, it is before you miss your turn. Your turn consists of moving tiles. You either play the tiles on the game board or exchange for new tiles. After you have completed replacing the missing tiles, you turn ends. A third alternative is simply to pass and not move any tiles.
If you can make a word, you can play all seven of your tiles in one turn.
Forfeit a turn.
A bingo is a play in Scrabble in which a player uses all seven tiles in one turn.
Each Scrabble (R) player draws seven tiles to start. And after each play, a player replaces any tiles used or traded so he always ends his turn with seven tiles (excepting near game end, when the tiles run out). For complete Scrabble rules, see the Related Link.
You can always replace any tiles during a turn by skipping your turn and selecting new ones from the tile bag ... but I think you're asking if you can play both blanks in a single turn. Yes, and those are scored as 0 points and will remain whatever letter you announce them to be during that turn in completing your word.
You get 50 point bonus for using all seven tiles in one turn in Scrabble. This is in addition to the score you earned for that word. So if you earned 25 points for the word, plus 50 for using all your letter tiles in that word, then you will earn 75 points.
Each player starts with seven tiles. If you can make a word, you can use all seven on your first turn.
Swapping tiles is an option that you have each turn in Scrabble. Instead of making a word, you can choose letters in your hand that you don't want, and trade them for mystery tiles that are still available. Of course, you won't get any points that way, and so making a low-scoring word that gets rid of some of your unwanted tiles is sometimes a better option. If you can't make any words though, then swapping is the best choice.
tap the tiles
If you use all seven of your tiles on one turn it is called a bingo. You get an extra 50 points on top of your word scores.
You can keep the tiles in a bag and draw when you need new letters from your turn. Another option, which I like, is to dump all the letters into the game box lid and draw the letters from there. If you use the lid, turn over and mix the letters before you start to play.