Having just read On Guerrilla Gardening: A Handbook for Gardening without Boundaries, I am feeling totally inspired and have started to organise an event for this magical day here in Sheffield.
There is a thread on the Guerrillagardening.org forums, and a facebook group for the event. Sign up for either of those to keep up to date with the plans, or keep an eye open here for news.
What you will need:
A bottle of water.
Something to make holes with (pencil, screwdriver, trowel, or in my case, a crutch!).
What to do:
Dig a small hole about 2cm deep, drop in a seed. Cover with soil. Water.
That's it! As many times as you like, in as many places as you feel will be cheered up by a glorious sunflower in a couple of months.
I've been stocking up on sunflower seeds at various bargain shops, if you grab a pack or two each time you see them, you should have plenty by the date.
There will be people meeting somewhere central to go and garden together, and others individually planting seeds where they are. Wherever you are, Sheffield or not, join in. Just imagine how happy you, and others, will be when there are unexpected sunflowers around!