Mazel (mah-zil) literally means "luck" and tov (tuhv) translates as "good", so the phrase literally means "good luck".
But in actual usage, it means "congratulations".
In other words--don't say mazel tov to an actor before a show--say it after the performance!
Don't say it to someone before they take the SAT, say it after they make a perfect score!
Don't say it to someone before they go on trial--say it after their lawyer gets them off!
Use mazel tov at weddings, graduations, on hearing about someone becoming a grandparent, or adopting a cat, or at any other joyous event.
And you can wish it to me today, as I celebrate my "blogiversary". I started this mish-mosh of a blog two years ago today!