I think Talmud Megillah mentions the various arrangements for the 4 parshiot. This year, and next, we find a break between Sheqalim and Zachor as well as a 2nd one between Zachor and Parah. I understand the placement of each of the other parshiot, and what would cause the breaks, possibly.
However, I don't understand why Parah must always precede Hachodesh with no break, ever. Can someone explain why these two must be together?