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Traders in the town said they had experienced a boom in business, with some doubling their intake on a normal day's trade.
Ailsa Austin, of Mere Moments, said: "It was a good business opportunity and it would have been silly not to open.
"Everyone was enjoying the community spirit. I even asked one of the customers if he wouldn't mind waiting while I watched the parade!"
Sharon Loome said the Mere Street café had taken the same amount of money in four hours as they usually take in eight hours of opening.
The success on the day might even prompt occasional new Sunday openings.
At the Sugar and Spice patisserie, which is usually open on Sundays, 10am-3pm, owner Gill Harvey was also pleased with trade on Carnival day.
She said: "We were mobbed on Sunday. I think it's a good thing for business."
At the Chip King's Corner Diner, owner Chris Hickling said anything which encourages people to come into the town must be a good thing.
"We don't want a party in the park, we want a party in the town," he said.
"It brought people slap-bang into the middle of town and that's a brilliant idea. Even if it's raining, people can go into the shops."
But although businesses were happy with the extra crowds on Carnival Sunday, most did not think the experience would encourage them to open on a Sunday in general.
Mr Hickling said there were not usually enough people in town to warrant it, although he would consider it if requested by customers.
Chix Clothing, which opened on Sunday for two-and a-half hours, will not be making a habit of the occasion.
"I think it was quite good really, but I don't think enough shops are open on a normal Sunday to make it work," said business partner Rod Dennis.
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The full article contains 352 words and appears in Diss Express newspaper.