Christians observe a fast on Friday, and so they don’t eat anything with meat. Why is Friday the day of the fast? Christians believe that Friday is the day of penitence, because Christ died on this day. Not eating meat on a Friday has long been a church rule, though it’s a difficult rule to observe given how hectic modern life is. The church authorities have amended this duty so that believers can continue to refrain from eating meat on a Friday if they want, or they can set themselves some act of penitence as they see fit, either a fast, or a pious act, or charitable deed.

Fast: To refrain from consuming meat, alcohol, sweets, light entertainment programmes on television, etc.
Pious act: going to mass, reading the Bible, the stations of the cross, praying with a rosary, the study of Christian doctrine, meditation, etc.
Charitable deed: Visiting and caring for ill people, helping those close to you, a specific service to a civic or religious community.

 
See this page in Czech