In biblical times, all the way back before the Babylonian exile, and possibly even during the wanderings in the desert. Passover was one of the 3 pilgrimage holidays, when all Jews (or at least the men) were to go to Jerusalem. On the eve of Passover, before sunset, each family was to bring a lamb to the Temple for slaughter, and then take most of the meat home for dinner. The Seder meal began as the festive meal where the Passover sacrifice was eaten "with unleavened bread and bitter herbs." After the destruction of the Temple in Jerusalem, the Seder meal became a way to remember the sacrificial system, and over the years, a fixed liturgy for the meal was hammered out and formalized, remembering not only the meal itself but the discussion among the Pharisees about how to celebrate the meal.
Reform Jews have only 1 seder instead of 2.
The theme of the Seder meal is freedom from slavery, based on the verse in Exodus "And you shall tell it to your son on that day, saying, 'Because of this God did for me when He took me out of Egypt'" (Exodus 13:8). It is a time of celebrating God's deliverance of the Jews from bondage to Pharaoh in Egypt.
One person who is alone is able to make a Seder. There is no minimum number of people, although it would be nicer is you had other people at the Seder celebrating with you.
Generally it occurs only during Passover, a Jewish holiday. There is also a seder for the holiday of Tu Bishvat, but this seder is rarely observed.
Yes
The word 'seder' is Hebrew for 'order'. The Jewish Seder is the ceremonial meal in Passover, during which, according to a set of ancient customs and text, we recount the Exodus from Egypt.
The Jews eat the Passover Seder meal on the night of Passover (Pesach). It makes little difference whether Passover begins on Shabbat or on a weekday. Note that there are festive meals on every Shabbat, but they are not called "seder" and the foods are different.
The Haggadah contains the service for the seder.
By celebrating it
It is speculated by some that the Jewish people were celebrating the Passover seder during the Last Supper, but this is disputed.
At the Seder meal we retell the story of the Exodus from Egypt.
Outside of Israel, Orthodox and Conservative Jews have 2 seders, but Reform Jews only have 1. Inside Israel, everyone has one Seder.