It also makes sense that election years would be positive as each administration pulls out all the stops to make sure the economy and stockmarket are positive when re-election time arrives.
That in turn usually results in the economy being overheated, and the stockmarket being over-valued again, and the cycle repeats, with the next administration then allowing those excesses to be corrected in the first two years of its term.
The most consistent longer-term pattern I have ever seen is that since at least 1918 the stockmarket has experienced a substantial rally from the low in the 2nd year of every presidential administration to the high in the following year.