[floatrock]

Interestingly, the voyager missions were also timed-events -- they were launched when they were because JPL realized it was a 1-in-175-year alignment of the outer planets that made it feasible to launch just a few crafts to visit the outer planets all in one go: https://en.wikipedia.org/wiki/Grand_Tour_program

[f6v]

AFAIK many launches are timed to make the flight shorter. Although not on 1 in 175 years, but shorter windows.