By not using an apostrophe in “Veterans,” you’re principally merely using the phrase as an adjective that modifies “Day.” For example, in case you say the phrase “pumpkin farm,” “pumpkin” describes the type of farm you’re talking about. On this case, “Veterans” merely tells you the form of day being spoken about, which is “Veterans Day.”
https://www.distractify.com/p/why-does-veterans-day-not-have-an-apostrophe | Why Does Veterans Day Not Have an Apostrophe in Its Title? Particulars
