Does TOS United Earth still have distinct, self-governing Countries such as USA, Russia, Canada, France, etc.? Maybe Earth countries are about the "same" as today, but under the umbrella of a stronger United Nations governance (now called United Earth).
For example, Chekov displays that Russia is his home country more strongly than UE. His education system is not EA uniform as it comes to language (he is primary Russian speaking with English as a second language), and its history education is very pro-Russian. Another example is that Scotty is a proud Scotsman from Scotland.
Back to turkeys and an USA Thanksgiving Day (and Christmas parties, too), I can easily see that the crew of a ship will celebrate their cultural holidays. The Enterprise appears to be crewed by mostly humans with Earth-based origins. Bringing a little fun and home to dreary, deep space life is good for morale.