I think this is more about Schelling points than the bystander effect. "Everyone agrees to have Shakespeare parties at time X of the year" is a Nash equilibrium for every X, so in practice the easiest way to decide on an X is if some particular X stands out in some way. This is why Christmas, Thanksgiving, etc. are nice; otherwise an entire family (not to mention their employers!) would have to agree on what time of year is a good time to meet, etc. There may also be a scarcity effect involved in only one X standing out and the feeling that it would be a waste to let it pass.

Corollary: if you want to start having more interesting themed parties, look for more interesting dates to put them on. History-themed parties pegged to the dates of important historical events seems like a good place to start. (Followed by fictional-history-themed parties pegged to the dates of fictional important historical events?)