In English, when we say "sacrifice", we mean that you
have given up something you wanted in order to get something else, like
you sacrificed your best piece of candy in order to get your friend to
tell you his secret. In Greek, sacrifice was a much more specific thing:
it means killing a tame animal and offering part of it to the gods, or
to one specific god, while eating the rest of it yourself.
We don't know when the Greeks first began to sacrifice
animals to their gods, but probably not long after
they first began to keep tame animals.
It was one thing to hunt and kill wild
animals like deer, or fish, but it bothered people
to kill animals they had taken care of, that trusted them. (And it still
bothers many people today.)
So they began to kill tame (domestic) animals only
when they thought a god wanted them to do it. And only all in a group,
all together, sharing the guilt.
Generally a sacrifice went something like this: somebody
decided that he or she owed something to a god, or that he or she wanted
something from a god. Or there were also regular times of year for sacrifices.
It's a lot like prayer: people today pray at certain times of year, and
maybe every Sunday morning or Saturday morning or Friday evening, but also
when someone is sick or before an exam, or after someone gets better.
Once you had decided to sacrifice, you called together
a suitable group of people to participate. Many sacrifices were family
matters, like if someone was sick,
and perhaps one or two chickens might be sacrificed with the family standing
around, and then the family would eat the chickens for dinner. There was
a stone altar outside each house that you could kill the chickens on, with
an appropriate ceremony. Then the fat and bones were offered up to the
god, while the meat was roasted over a fire.
(Yes, it bothered the Greeks that they ate the good
part and gave the god the yucky part. The myth
of Prometheus tries to explain why this
is).
Other sacrifices were larger: a goat, a sheep, or
a pig, or even sometimes a cow. To sacrifice a goat, you'd have to get
together more than just your own kids: maybe the whole block, or
your cousins and their kids. All the meat had to be
eaten at
once (partly for religious reasons and partly because
there was no refrigeration!). Often sacrifices
involved more than one animal, and the whole village or the whole town
came to participate in it. This helped to bring people together, by
showing you who was part of your group (we have potluck
school dinners and end-of-season sports banquets and so on in the same
way).