Nearly 700 runners who completed the 6-mile run were left perplexed after they were handed their medals at the finish line in Market Street.
The wrong date - August 28 - had been engraved on the medals, prompting the Council to apologise for the error.
In a post on Facebook, the Council explained that the "pandemic has caught us out".
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A Council spokesman said: "We’re sorry about this - all the changing of event dates due to the pandemic has caught us out. It was a genuine mistake. We hope everyone enjoyed the day."
But many of the runners who completed the mini marathon have been forgiving, saying the typo "is not the end of the world".
Louise Gill, who is recovering from a 15-month battle with 'Long Covid', took part in the 2K Family Run.
She said: "Who cares? After the rubbish of having Covid and being unwell for 15 months, I don't give a hoot.
"Had a fab day and I'm looking forward to next year. So thanks Chorley Council. "
Karl McLaughlin added: "Well done Chorley Council and Fylde Coast Runners. Very well run event again and the support shown by the town was spot on."
Lucy Seddon, who completed the 10K, said: "Had a fabulous day and the medals will be worth more in the future. Roll on next year!"
But some folks appeared less forgiving.
"What a woeful mistake," said Liz Moon. "It makes Chorley look stupid. Thankfully it does not take anything away from those inspiring folk who took part."
Simon Croft, 40, of Red Rose Road Runners finished first in the 10K with a race time of 33 minutes and 49 seconds - two minutes ahead of 30-year-old Daniel Miller who came in second place.
You can find the results of the Chorley 10k here.