The first day is over, and I have an almost playable game. The aliens, check. Advancing menacingly towards you, check. Shooting them, check. End game when all killed or when they reach you, check. A saucer at the top, check. Dropping bombs at you, check.

What's still missing: game over/you win screens, bombs killing you, multiple lifes, score, the actual mechanic that is novel (secret).