Fish and chip Fridays – the perennial delight of schoolchildren universally – are thanks to an ancient Catholic tradition.

The Holy Grail of canteen lunches originated with the Catholic practice of abstinence where devout believers refrained from meat on every Friday throughout the year.

Under the Code of Canon Law for the Catholic Church Christians are bound 'to do penance each in his or her own way'. But 'in order for all to be united among themselves' the law goes on to outline 'penitential days' where all Christians are supposed to unite for a special devotion.

These days include every Friday throughout the year and the season of Lent, although some bishops have now downgraded that to just Fridays in Lent.

Fridays are particularly significant because they are remembered in the Christian calendar as the day Jesus was crucified.

As one Catholic blogger wrote: '"Abstinence" refers to the practice of abstaining from red meat, whether on Ash Wednesday and the Fridays of the Lenten season, or all the Fridays year round.

'"Fasting" refers to a specific practice of eating, during the course of a day, one regular meal and two smaller meals, wherein the two smaller meals don't add up to one regular meal. One can 'abstain' without 'fasting', but on Ash Wednesday and Good Friday, one does both.'

The weekly practice was designed to unite Catholics around humbly recognising their own sin and weakness.

But there are exceptions.

St Patrick's Day, a celebration, falls on a Friday in Lent this year. Two traditions of feast and fast run headfirst into each other, giving Catholic bishops a headache.

Church leaders in a number of dioceses have opted to waive the fast for this Friday, March 17, allowing Catholics to enjoy the traditional corned beef to mark St Patrick's Day.

'Many people have been asking if I will allow Catholics to eat meat on St. Patrick's Day since March 17th falls on a Friday of Lent this year,' David A Zubik, Bishop of Pittsburgh, wrote in a letter issued Wednesday. 'After much consideration, I have chosen to dispense Catholics in the Diocese of Pittsburgh from the obligation to abstain from meat on Friday, March 17.'

But Catholics aren't completely off the hook.

'I do appeal to those who choose to eat meat that day to do another act of self-sacrifice in the spirit of the Season of Lent with the mind and heart of Saint Patrick,' Zubik added.

In a subtle reference to the tendency towards over-exuberance, he encouraged Catholics to celebrate Saint Patrick in a manner that 'truly honors this good and humble saint.'