I always thought Mazel Tov meant "congratulations." I recently heard that it actually means "good luck." But I thought Jews don’t believe in luck…?
Your confusion is understandable. The Talmud–the ancient encyclopedia of Jewish wisdom–seems to contradict itself on the issue. In one place it states, "On your birthday, your mazel is strong." Elsewhere the Talmud reports, "The Jewish people are not subject to mazel"!
The word mazel literally means "a drip from above." Mazel can have different connotations depending on its context, but they are all connected to this basic definition–something trickling down from above.
The signs of the zodiac are called mazalot. Jewish tradition sees the constellations on high as directing the destiny of individuals and nations down below. Thus mazel is the influence dripping down from the stars. (Over the years, bad or good mazel came to mean luck more than destiny.) When the Talmud says that we are not subject to mazel, it means that we are not limited to our destiny; rather our own actions determine our fate.
There is another meaning of the word mazel that is more relevant to the phrase Mazel Tov. Mazel is the term used in Jewish mysticism to describe the root of the soul. The mystics say that only a ray of our soul actually inhabits our body. The main part of the soul, our mazel, remains above, shining down on us from a distance.
Have you ever experienced a sense of spontaneous intuition, where out of the blue you suddenly feel at peace with yourself and the universe? Or a sudden flash of inspiration that makes you see life in a new light? Occasionally we may receive an extra flux of energy from our soul above. It can happen at any time, but is most common at a time of celebration-–a birth, birthday, bris, bar/bat mitzvah or wedding. It is especially at these times of joy that we are able to see beyond the mundane and the petty and sense the deeper truths of life.
When we tell someone Mazel Tov, we are giving them a blessing: May this drip of inspiration from your soul above not dissipate, but rather have a positive and lasting effect, that from this event onwards you should live your life with higher consciousness. You should be aware of the blessings in your life and be ready to receive more and more.
In other words: Good Mazel!