Bar /Bat Mitzva

Literally, Bar or Bat Mitzva means 'son or daughter of the commandment'. But what we usually mean by it is the party, thrown to celebrate the fact that a boy or girl has reached adulthood according to Jewish law - meaning they have to keep the commandments.  For boys this is their 13th birthday (according to the Jewish calendar), for girls their 12th. From then on, for example, he has to say the blessing over the wine and she starts to light candles every Friday night .

Bar/Bat Mitzva parties vary from modest to extravagant. Their are no rules for this, since they will be bar mitzva at this age, whether we party or not. But from about the 15th century onwards it has become customary to have some kind of ceremony. Originally this consisted only of the boy (girls didn't count in those days) being called up to read from the Tora in synagogue, or at least to say the blessing over the reading of the Tora.

Nowadays this usually happens on Shabbat, so that everyone will be free to come. It can be done on a Monday or Thursday as well however, when the service also includes a reading. For girls the customs vary according to the religious flavour of their synagogue.

Reading from the Torah is no simple matter since two handy reading helpers are completely lacking: there are no vowel signs and the signs indicating the melody at which to read are also lacking. If it weren't for the pointer, you could easily loose your way.
Usually the bar mitzvahboy-to-be starts practicing a year in advance, because it can be pretty daunting to stand up in front of all your family and friends and read faultlessly like you should. The reading has to be just so and every mistake has to be corrected by the community. So there you are, layening without any of the ordinary tools, while the whole congregation is sitting their with texts that do contain vowel and melody signs...But afterwards there's the party. Usually the boy or girl gives a speech after the service or at the dinner, but this is a mere snip compared to what went before.

Whatever else happens, there is usually a kiddush in shul; often there is also a reception there, in a hall or at home, and a dinner later on. The more elaborate the festivities, the more people are invited.
People who don't normally go to synagogue - or at all, because non-Jewish friends and business contacts are often included - can be very uncertain about what is expected of them. In fact this is really quite simple; if men and women are seated separately you make sure you end up in the right section, and after that you just get up or sit down when everyone else does. After the reading, just 'congratulations' or mazal tov to the boy and his parents will do nicely.

And as for the party: why, that's just a party. Enjoy!