Does the phrase "Sankta Tago de Zamenhof" mean that it is a holy day for Zamenhof himself but not necessarily for other people?
The word de has a lot of usages in Esperanto which can sometimes lead to ambiguity. The level of ambiguity varies depending on the structure and the context.
- ĉapelo de Zamenhof - the hat belongs to him.
- libro skribita de Zamenhof - he is the doer.
- la lingvo de Zamenhof - the language came from him.
I don't find the sentence Sankta Tago de Zamenhof to be particularly ambiguous. From the context of our understanding of what "holy days" are, it would seem that we're talking about a holy day given in Zamenhof's honor.
For the record, I don't think there is such a day. The 15th of December, for example is a "festotago" but not a "sankta tago."