As well as encouraging schools and workplaces to raise money on a hat wearing theme, the charity is running a celebrity hat auction, which starts tomorrow.
Sports stars, actors, comedians and musicians are donating headwear. Donations are already confirmed from Helena Bonham-Carter, Eric Clapton, Sienna Miller and Alexei Sayle.
Bidders also have the chance to own an original Susan Hilferty-designed hat worn by the Wicked Witch of the East, Elphaba, in the Broadway production of Wicked. If you've always longed for a piece of designer millinery, get your bids in for lots by Stephen Jones, Philip Treacy and Lock and Co.
To entertain us all until the auction opens, if you can come up with some good celebrity/hat puns, perk up this thread on Woolly Hat Day's Facebook page and follow @WoollyHatDay for news on celebrity hats.
Make a note in your diary to wear a woolly hat on Friday 4 November. If hat-wearing is a no-go for your barnet, make a Woolly Hat Day donation online now.
Established in 1969, St Mungo’s provides housing and support to more than 1,700 people each night in London, Oxfordshire, Reading, Bath and Bristol. Its vision is that everyone should have a decent place to live, something meaningful to do, and satisfying relationships with people – as well as the good health to enjoy them.