March 4th is the annual celebration known as GM's Day.
To celebrate the day, all many publishers and online retailers are offering discounts. For details check out the ENWorld news page.
"To celebrate the day, all EN Publishing PDF books will be on sale at $1 each from Saturday 28th February until Saturday 7th March. Click here to view EN Publishing's PDF catalogue, and pick up a gift for your hard-working GM (or even something for yourself)."