We typically schedule tasks on regular intervals, e.g. P1D (every day) or PT1H (every hour), however, sometimes our intervals are irregular. In the extreme, we might have a list of completely arbitrary dates to work with.
We can implement such workflows using Jinja2 to write out the recurrences we need. There is now a new example in the Cylc documentation showing how to do this, including a couple of tricks for working with irregular inter-cycle dependencies.