yeah it's tough.

to celebrate something means to engage in ceremony or festivities, and ceremony means "A formal act or set of acts performed as prescribed by ritual or custom"

so celebrating something usually means purposefully doing something with the particular event in mind. for instance, as thatdarngirl mentioned with thanksgiving: the typical ceremonies of thanksgiving include eating a turkey dinner, getting together with family, but it also includes the whole pilgrim thing, native americans, history, nationalistic pride and stuff like that.

so, does eating a turkey dinner with family on any day of the year mean you are celebrating thanksgiving because that's one of the thanksgiving ceremonies? or is it really the motivation and reason for having the dinner what makes it "celebrating" the holiday?