So as you've probably come to realize the "Day" in the title really isn't significant. I could be "Issue", or "Section", or "Chapter" if you like. It's really just a way for me to designate each subgoal within the project. And that's really important - subgoals. When you're making a game, the entirety of the game itself can be incredibly overwhelming. Menus, scores, multiplayer support, weapons, animations, the list goes on and on and on. And a lot of times you're going to be thinking - what next? How do I get it all done?
Your ability to compartmentalize your game into sections - and more importantly - systems, is going to be one of the most valuable skills for you to develop. Pick out a specific section of the game, decide what the goals are for that section, and implement it. But all of the pieces of the game are intrinsically interconnected. And most pieces build upon some other piece. Which is why it's not only important to decide which section you're working on, but what order are you going to build the sections of your game. For instance, I wouldn't spend a huge amount of time focusing on building a level in my game until I had a better grasp on the gameplay - because those things are going to shape level design.