Leviticus appears to be arranged in a chiastic order (see here). That is, it is arranged in a mirror-image. This is best understood visually.
Notice that the first and final chapters match. These both describe rituals, whether sacrifices and offerings (Lev 1-7) or festivals (Lev 23-25). The next chapters, moving inwards, are also parallel: they both address the ordination of the priesthood (Lev 8-10) and the moral requirements of the priests (Lev 21-22). Moving inwards we find, ceremonial purity (Lev 11-15) and moral purity (Lev 17-20). This brings us to the final and central unit, the day of atonement, which finds no parallel within the book. This means that it is the centre of Leviticus itself. In fact, the Day of Atonement is the centre of the Torah, given that Leviticus is the central book (another chiastic structure).
My wife drew this amazing promo picture for the class I am teaching at CCB. She built it around the chiastic structure. The wheat and birds match the food on the right, showing the Ritual section. The anointing oil and blood of the ram match the bald and disqualified priest on the right in the Priesthood section. The grasshopper and fish match the unequal scales on the right in the Purity section. Then finally in the centre we have the Day of Atonement. Amazing, right?
This results in a seven-fold structure (a number that occurs frequently within the book) that is easy to memorize. For such a complicated book, we need all the help we can get!
Randy (over at Bible Study with Randy) and I just put up an episode about chiasms on Beyond Reading the Bible.
