The grassroots campaign goal is for millions of students to try an Hour of Code during December 8-14, 2014, in celebration of Computer Science Education Week. Is it one specific hour? No. You can do the Hour of Code anytime during this week.
To get started, try a tutorial.
Last night, I worked through the Angry Birds programming tutorial with one of my daughters. It was pretty fun. To move the bird, you need move and drop programming instructions that are displayed interlocking blocks, kind of like 2D lego blocks.