Yom Kippur (the Jewish Day of Atonement) shouldn’t be confused with kipper (a fish that is often eaten for breakfast), but there are jokes.
 
“Celebrating this Jewish holiday with a fish breakfast; yum kipper!” was posted on X/Twitter by Neil Gibson on September 28, 2009. “Tasty jewish breakfast : yum kipper…?” was posted on X/Twitter by Ken Dodd Nagasaki on May 29, 2011. “‘Yum, kipper!’ - a Jewish man enjoying his breakfast on the Day of Atonement” was posted on X/Twitter by Scott Hoad on October 22, 2013. “What does a Jewish pescatarian say at breakfast time on the day of atonement? ‘Yum, kipper.’” was posted on Sickipedia on July 25, 2023.
 
 
Wikipedia: Kipper
A kipper is a whole herring, a small, oily fish, that has been split in a butterfly fashion from tail to head along the dorsal ridge, gutted, salted or pickled, and cold-smoked over smouldering wood chips (typically oak).

In the United Kingdom, Republic of Ireland and some regions of North America, kippers are most commonly eaten for breakfast.
     
Wikipedia: Yom Kippur
Yom Kippur (/ˌjɒm kɪˈpʊər, ˌjɔːm ˈkɪpər, ˌjoʊm-/; Hebrew: יוֹם כִּפּוּר, Yōm Kippūr, [ˈjom kiˈpuʁ], lit. ‘Day of Atonement’) is the holiest day in Judaism and Samaritanism. It occurs annually on the 10th of Tishrei, corresponding to a secular date in September or early October.
               
X/Twitter
Neil Gibson
@Neil_Gibson
Celebrating this Jewish holiday with a fish breakfast; yum kipper!
10:52 AM · Sep 28, 2009
   
X/Twitter
Jude Rawlins
@juderawlins
Breakfast can be quite ceremonial. Yum Kipper.
7:59 AM · Jul 19, 2010
 
X/Twitter
Ken Dodd Nagasaki
@Suspiriac
Tasty jewish breakfast : yum kipper…?
7:04 AM · May 29, 2011
 
X/Twitter
Roberto Ferdenzi Ⓜ️ 🇮🇹 🏆
@RobertoFerdenzi
@FoxxTailSoup Had the holiest of Jewish breakfast’s today - delicious smoked herring….......Yum, Kipper!
4:29 PM · Dec 1, 2012
 
X/Twitter
Tom Appleton
@thetomappleton
Happy Yom Kippur to my Jewish friends and happy “yum, kipper!” to all those eating fish for breakfast. 😊
4:57 AM · Sep 13, 2013
 
X/Twitter
Scott Hoad
@ScottHoad
“Yum, kipper!” - a Jewish man enjoying his breakfast on the Day of Atonement.
6:40 PM · Oct 22, 2013
 
Reddit—ImagesOfNew York
Posted by u/ImagesOfNetwork August 10, 2017
[I Ate] Yum Kippered at Russ & Daughters in NYC (Kippered Salmon and a Bagel)
i.imgur.com/djx5pk…
   
Reddit—Jokes
Posted by u/ElderCunningham October 9, 2019
What did the yenta say when she finally bit into her herring, after fasting for 24 hours?
“Yum, kipper.”
 
Sickipedia
chum
25 July 2023-Other-Stupid
What does a Jewish pescatarian say at breakfast time on the day of atonement?
“Yum, kipper.”
 
X/Twitter
Bob Kostic
@causticbob
What does a Jewish pescatarian say at breakfast time on the day of atonement?
“Yum, kipper.”
6:52 PM · Jul 25, 2023