Cardiff face Newport in Martyn Williams' honour
Williams, whose younger brother and mother died of cancer, will use a series of events to raise money for the NSPCC and Velindre Cancer Centre.
The occasion will mark the first time the Blues and Dragons have met in a friendly, but they have clashed 14 times in the Magners League with the Blues winning ten and the Dragons four.
The 34-year-old, who has 95 Wales caps to his name, is gearing up for the start of the new season after being rested for summer fixtures against South Africa and New Zealand, with head coach Warren Gatland desperate to ensure his first-choice openside is in the best possible shape for next year's World Cup.
He will also be looking to help his region build on their Amlin Challenge Cup victory of last season. Williams said: "I feel very honoured to be awarded a testimonial game and really grateful to Bob Norster, Peter Thomas and Dai Young for arranging it. I'm not aware of Cardiff or the Blues ever arranging a fixture for a player before so it's very humbling to be granted a match.
"When I first joined Cardiff our biggest rivals were always Newport. They had the some real quality players like Shane Howarth, Jason Foster, Gary Teichmann, and Percy Montgomery and they were some of the toughest games of my career.
"The atmosphere, because they were local derbies, was always brilliant so to play another Welsh region in a pre-season friendly is a bit special. It's on a bank holiday weekend so hopefully the sun will be shining and we will have a big crowd down.
"The Dragons were one of the most improved sides last season and they've got a great squad down there which they've strengthened during the summer. In a way they are a side you never want to play against because they always make it difficult for you."