The day, held annually on August 1, celebrates all things Yorkshire.
But Pontefract-based gift firm Love Layla has now designed a cheeky card to celebrate the day by having a pop at the county's historic rivals and neighbours Lancashire.
The two counties fought out a bitter and bloody series of civil wars for control of England in the 15th century.
Despite the fighting ending in 1487, the neighbouring counties have enjoyed a light-hearted rivalry ever since.
And now Lancastrian pride has been wounded again by a card that reads in Yorkshire dialect: "Proper chuffed tha’s not from Lancashire".
Founder Stacey Dennis said the card has, ironically, "sold like Eccles cakes" ahead of the day.
Stacey said she wanted to develop the range to mark the occasion and also proudly celebrate her own Yorkshire roots.
She said: “I wanted to do something that would really help the people of Yorkshire to mark this Yorkshire Day in style, especially after the last 18 months we’ve endured.
"I thought that designing a range of cards that celebrates Yorkshire humour and one that pokes fun at our friends across the border would be a great way for the whole of Yorkshire to get behind what this great county is all about."
Stacey, who founded Love Layla six years ago, started printing her cheeky designs in her bedroom after being made redundant as a graphic designer.
She now has a multi-million-pound business, a warehouse operation in West Yorkshire and a business in Australia.
But she is no stranger to controversy and sparked outrage in 2019 with a range of Christmas cards that read: Mary just needs to admit she slept with someone else."
Stacey added: “We’ve got so much to celebrate in Yorkshire and can boast of some fantastic global exports from the Bronte Sisters and Sean Bean to Wensleydale cheese and Yorkshire Terriers, one of the most enduringly popular dog breeds in the world.
"We’ve even hosted the Tour de France. Lancashire can’t hold a candle to what Yorkshire has given the world, although I bet they’d beg to differ. That’s how wars start!”