As an example, Thanksgiving (US) is celebrated on the 4th Thursday of November. I'm looking for a way to describe that situation in a general sense. As in "Thanksgiving is a holiday that occurs __" or "Thanksgiving is a __" holiday. Monthly is close in a sense, although it doesn't quite capture the relative placement of the event. I'm also wondering what other words are associated with the concept. If there isn't a way of describing it well, I'd also be interested to hear ideas for coining a word for this.
For the curious, the context that this question arose from was in describing the manner in which one can schedule events. The title says "Monthly (Day-Relative)" with the subtitle "Choose the 1st, 2nd, 3rd, 4th or 5th day of week or last day of month the event should occur". Day-relative seems to be an OK candidate, although it doesn't feel like it describes the situation completely.