There is no establish tradition of having an empty chair at a seder. There are some groups today who have an empty chair at the table in honour of friends and/or family who have been killed in war or who are in the military and cannot be present at the seder.
The actual tradition is to have a full glass of wine on the table during the seder. This cup is for the prophet Eliyahu who is said will arrive before HaMoshiach does to anounce his arrival and anoint him.
It's for all who are hungry can come and eat.
Seder = סדר
Some say that it has no special meaning, being just a container for the meaningful items. Others say that the plate itself represents Malchut (God's kingship).
In the seder meal the parsley, or other type of bitter herb, represents the bitter taste of slavery and affliction. In some homes parsley is used because it looks similar to a flail.
night of passover
No. A Seder is a Jewish religious meal recalling the Passover.
Sometimes.
Four glasses of wine are drunk as part of the seder.
At the Seder meal we retell the story of the Exodus from Egypt.
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.
One of the foods on the Seder plate is the Z'roa - a roasted shank-bone of lamb or goat, or a chicken wing, or chicken neck. It symbolizes the korban Pesach (Pesach sacrifice), which was a lamb that was offered in the Temple in Jerusalem, then roasted and eaten as part of the meal on Seder night. See also:More abut the Seder meal
At sundown.
The parsley is a green vegetable, representing spring-time and renewal; during the seder, it is dipped into salt water, to represent the tears the Israelites cried while living as Egyptian slaves.