People eat fish on Friday primarily due to a Christian religious tradition that commemorates the death of Jesus Christ, who is believed to have died on a Friday. Early Christians observed Friday as a day of penance and sacrifice, fasting or abstaining from meat to honor Christ's sacrifice. Since meat from warm-blooded animals was considered a luxury and symbolic of indulgence, abstaining from meat on Fridays was a form of penance and modesty. Fish, being cold-blooded, was considered acceptable to eat on these meatless Fridays. This tradition also ties to biblical symbolism and early Christian practices where fish had special significance. The practice became widespread in medieval Christianity and shaped dietary customs, especially during Lent, influencing even the development of the fishing industry in Europe. In summary:

• Friday fasting/abstinence commemorates Jesus's crucifixion on Friday.
• Abstaining from meat symbolizes penance and humility, as meat was a luxury.
• Fish was allowed because it is cold-blooded and symbolically different from meat.
• The tradition has deep historical roots in early Christian and medieval practices and persists in many Christian communities today.

This explanation is supported by several historical, religious, and cultural sources detailing the origins and reasons behind eating fish on Fridays.