The story of the evening of the Buddha's Enlightenment: he went into deep meditation on the concentration level, being aware of his breathing as the main technique. He developed psychic abilities, and he was able to see back in time, long periods of time, many of his rebirths. He was able to see the law of Cause and Effect, other people dying and where they got reborn, he was able to see with many different psychic abilities. But this wasn't enough to actually become enlightened, this was just the psychic ability. When his mind became fairly settled and not so attracted to all these psychic abilities, not involved in them, he started to contemplate Dependent Origination. He started looking at Dukkha and its cause. He went forward, he went backward, he looked across lifetimes, and he would look across moments. He investigated Cause and Effect over and over and over, this whole dependent chain. He realized the Four Noble Truths. This was a new found understanding, it was unknown to anyone prior to that. He understood that Dukkha is caused by our craving, and that there's a way to end Dukkha, by practicing the Noble Eightfold Path. When he penetrated this understanding, he realized that he was enlightened.
There was also the attacks of Mara. I may not get the order of how these sequences happened perfectly right, but it was kind of like this. His ignorant part of his mind, what we call Mara, wanted to attack, "No, I'm not going to let this person get enlightened, I'll be destroyed him, forget it!" Mara evidently threw him lots of fearful thoughts, armies coming to attack, whatever else. The Buddha didn't get afraid, these thoughts were not going to bother the Buddha at all. Mara tried sex, visions of naked women, whatever else. The Buddha wasn't interested, he had lost all desires, it was done. Mara had thrown in two big powerful ones, and the Buddha didn't even move. Mara threw the last one, the BIGGIE. Those were just the two big powerful ones; then Mara threw the BIGGIE at the end. Doubt! You've heard me say it, doubt is the hardest of the Five Hindrances, it's the big one. Mara says to the Buddha, "What right do have to think you're enlightened!?!" Ah, doubt! But the Buddha knew he had perfected the Paramis, and said, "Let the earth be my witness." All ten thousand earth systems and world cycles everywhere in the universe shook with an earthquake. Mara disappears! That's it.