Organisers of the festival - being held at Williamson Park - have announced a fourth night on Sunday, September 5.
Headliner Rick Astley and special guest Sophie Ellis-Bextor will perform on the night, along with support from the Cuban Brothers.
And 1,000 free tickets for the Sunday night will be given to NHS staff as a 'thank you' for their hard work over the last 18 months.
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Tickets for the additional night go on sale at 10am on Thursday but NHS workers can apply for the free tickets now, with the ballot open for 48 hours ahead of general release tickets.
To register for the ballot visit here.
Sunday will now close a huge weekend of live music at Lancashire's biggest open-air music festival, which is set to welcome 35,000 ticket-holders for four days of musical performances from some of the music industry's biggest names.
Originally set to take place in May 2020, the festival will run from Thursday, September 2 to Sunday, September 5.
More than 50 acts and artists have been announced, with Thursday featuring James, The Lightning Seeds and Get Cape Wear Cape Fly.
Friday will feature DJ sets from the likes of Rudimental, Wilkinson and Crazy P Soundsystem, plus Becky Hill and Ella Eyre, while Saturday will see Rag'N'Bone Man headline with acts including Lancaster band The Lovely Eggs, The Lottery Winners and The Futureheads all on the bill.
The festival has also announced that The Big Family Day out is back for its third year at the festival during the day on the Sunday, with festival-style activities designed for kids of all ages.
Festival director Richard Dyer said: "We're going to throw the biggest, most outrageous party of 2021. We cannot wait for you all to see how we plan to transform the beautiful Williamson Park once again, into an amazing, sensory overloading festival site."
Sunday tickets go on sale from 10am on Thursday at www.skiddle.com. A limited number of tickets for the other days are also available.