Last Spring my family and I moved across the county from the foothills of Loveland, Colorado to Hudson, Wisconsin on the banks of the St. Croix River to join the community at Beth Immanuel.
Upon arriving, we promptly received several Friday night Shabbat invitations. As we ate sumptuous meals at various homes we noticed that fish was a big theme. For one thing, everyone has a “Gurgle Pot” in the shape of a fish. When you pour water from the spout it makes a gurgling sound.
Second, in many of the homes the meal began with Gefilte fish. Gefilte fish is a poached mixture of several kinds of deboned fish. Now, according to my family (except for me) there was never anything appetizing about this appetizer. You see, the only kind of Gefilte fish with which we were familiar comes in a jar and sits in some kind of fish juice. I would happily slather this onto crackers and matzah but as for the rest of my family, not so much.
This all changed when we discovered the frozen loaf type of Gefilte fish that you cook and served sliced. If you haven’t tried it I highly recommend it. Now that my wife has mastered making this, together with our own Gurgle Pot, Gefilte fish has become a staple (well at least for one month now) at my family’s Shabbat table. We usually serve it with cooked carrots, pickled beets, pickles, and/or olives. We garnish the fish with sauces such as horseradish or sriracha mayonnaise. All in all, it’s simply delicious. A lot better than the jarred stuff, I can tell you that.
Yet, this custom is not just unique to Beth Immanuel. Fish is a staple on Shabbat for Jews throughout the world. Why? Well, as usual tradition offers several different answers. Rabbi Shmuel Kogan gives a practical reason:
The prophet Isaiah (58:13) tells us, “and you will proclaim the Sabbath ‘delight,’ and the holy [day] of G‑d ‘honored.’” Our Rabbis explain that one honors the Shabbat by wearing special clean clothes and has pleasure on Shabbat by enjoying delightful food and drink on this day. In the days of the Talmud, the food that was considered delightful and was used for this purpose on Shabbat was fish. Meat and wine were added to the Shabbat meal since most people enjoy these foods too. One can and should eat any specific Kosher food that they consider pleasurable.
Other sages give more spiritual reasons. The disciples of Rabbi Simcha Bunem of Prysucha related:
Our master—may his merit protect us—said that the custom to eat fish as the first food on the holy Sabbath is based on the fact that fish were the first created living beings, and the holiness of the Sabbath, too, is the root of life; for this reason Israel has the tradition to start delighting in the Sabbath with fish food. 
As disciples of the Master, I think we have some additional reasons to partake in fish on Shabbat. First, many of the Master’s disciples were fishermen and many of the gospel stories involving food make mention of fish. The Master even uses fish as a metaphor for the job of a disciple: “Follow me, and I will make you become fishers of men” (Mark 1:17). Additionally, Jewish tradition teaches that one of the foods of the Messianic Banquet will be the Leviathan, the giant sea monster. The Sabbath is a taste of the Messianic Kingdom and the meals we eat together on the Sabbath are a taste of the Messianic banquet. Therefore, how fitting that disciples of Yeshua eat fish on the Sabbath as an act of connecting (devekut) to the Master as we long for his return when he will usher in the kingdom of heaven.
At any rate I think it’s a meaningful (not to mention delicious) custom. It doesn’t have to be Gefilte fish. It could be herring, salmon, or ahi tuna. The point is that we are tasting something physical to remind us of something spiritual. We are tasting today of the
bread fish of tomorrow. May our Sabbaths be filled with peace and the fishy presence of our Master!
- Rabbi Shmuel Kogan, “Why Is It Customary to Eat fish on Friday Night?” n.p. [cited 9 September 2016]. Online: http://www.chabad.org/library/article_cdo/aid/531483/jewish/Why-is-it-customary-to-eat-fish-on-Friday-night.htm
- Eric G. Freundenstein, “Sabbath Fish,” n.p. [cited 9 September 2016]. Online: http://www.myjli.com/why/index.php/2016/02/05/sabbath-fish/
Source: First Fruits of Zion