Pesach Seder
Question
Why do we spill wine at the Seder?
Answer
Shalom!
Thank you for your question.
Indeed, it is virtually a universal custom to spill ten drops of wine from one's goblet when mentioning the ten plagues at the Seder. As the Rema writes, "It is customary to spill some wine with one's finger at the words ’blood, fire, and columns of smoke’ …and when mentioning the Ten Plagues, and at the words ‘D’tzach, Adash, B’achav’…for a total of sixteen times." In some families, only the one leading the Seder does this, and some have the custom for everyone to do so, as well, Hagaon Harav Fried Shelita says that someone who doesn't have a specific custom should do like this minhag
The reason wine is spilled at this time is to recall the revenge that God took upon the Egyptians on our behalf.
There are a number of customs on how to spill these sixteen drops of wine. The Mishneh Berurah writes to spill the wine by using their index finger in order to recall the "etzba elokim," the finger of God, which is said to correspond to the index finger. Although some have the custom to use the pinky finger to spill these drops of wine. There are also those who use the ring finger to spill the wine, based on the teaching that God's smiting of the Egyptians is represented by this finger. In a case were a person doesn't have a specific custom as to which finger to use, then one should use the index finger as brought by the Mishneh Berurah.
When removing wine from the cup, one should have in mind the hope and wish that God send all such plagues upon all the enemies of the Jewish people.
If a recognizable amount of wine was missing, then the wine cups should be refilled right away or at least before reaching the blessing of "…asher ga'alnu.
It is important to note: This year, some red wines are from the Shemita year, and since it has Kedushah to it, one should not spill the wine at all, not even when mentioning the ten plaques.
SOURCES: Rema, OC 473:7; Darkei Moshe, OC 473:18; Maharil, Seder Haggada; Magen Avraham 473:28; Aruch Hashulchan, OC 473:24; Mishna Berura 473:74,75; Be'er Heitev 473:26.; Siddur Harav.