Ever since, we have set aside this day to give special thanks to God for the many blessings, gifts, and love he has bestowed on us and our country.Thanksgiving in this country has a specific connection to an event, involving the Pilgrims and so forth, but the basic idea of a harvest festival or celebration has ancient origins. (The specific event is of limited importance in our traditions and when the holiday was set in the mid-19th Century and beyond, it became more of a general family holiday though we can honor the details.) This includes something that has a component in which we give thanks and at times there is also a special somber component ("as a day of national humiliation and prayer" etc.).
As with the official one cited, they often have a religious component. Early presidential thanksgiving proclamations provided a chance for an official statement with a religious character within the limits of our secular government. There were dissents -- Thomas Jefferson felt that such a move was a violation of the First and Tenth (since it was a power left to the states) Amendments. At the very least, it would seem to me that these things should be inclusive. The one this year is not. There is a way, especially since even many who believe in God do not think of the holiday as specifically a religious holiday, to include everyone here.
The presidential proclamation here provides one of various ways that the Trump/Pence Administration can "establish" their breed of evangelical tinged religious belief by official acts. It is rather minor as compared to let's say providing more exceptions to the contraceptive mandate or something, but I do think it notable. This holiday season -- perhaps we can think of it as a two month span in various respects (Halloween to New Years) has a lot of charming components (including Hallmark Christmas movies), but much diversity. We should honor that.
I have long focused some on holiday display cases as decided mainly by the Supreme Court. It is shall we say part of my holiday tradition with Oyez.com allowing me to listen to various oral arguments. My sympathies are with a separatist position such as spelled out by Justice Stevens: "The Establishment Clause should be construed to create a strong presumption against the installation of unattended religious symbols on public property." In practice, official mixture here will in the long run favor certain religions over others and there are many alternatives to some official creche or menorah etc. For instance, I recently saw reference to a set of four postage holiday stamps and there was really no non-religious option though it was pointed out Kwanzaa is in effect a secular holiday. Perhaps, but not one really of a general character. And, symbols do have special power.
Thus, the "passive" nature of displays only takes us so far. There also is a theme in these cases on one side that the test should be some sort of coercion or proselytizing (Justice Kennedy suggested a gigantic cross on a government building could be to much.). But, that has a free exercise character. The Establishment Clause goes further than that. Thus, "We are a Christian Nation" type proclamations might not be "coercive" though in practice there really is a long term overlap once a certain group is favored even if others do not directly are coerced to pray or something.
With new membership, the state of the law here might be in flux and a ruling involving a gigantic cross might provide further clarification. The current rule appears to be that a display can not be seen to endorse religion. This encourages the usage of diverse displays, a creche standing alone being found to be unconstitutional (if by 5-4). Stevens' opinion involved one type of case -- localities that tried to specifically not allow religious displays. This was seen as content/viewpoint discrimination, which I find somewhat dubious given the Establishment Clause provides a special case here. Interestingly, Justice White took a middle path here in a case involving student groups, another issue that arose repeatedly in some fashion. He would allow flexibility and that to me seems like a good idea.
To clarify here, the matter involves the government, not private display. It also doesn't involve simply having Christmas as an offiical holiday, which both honors its secualar character while also having a free exercise component (e.g., New York City made the Jewish New Year a school holiday for related reasons). The latter case involved a public area near the seat of government that provided a chance for various people to put different types of freestanding displays such as a thermometer stating money raised for some charity. The state didn't want to allow religious displays since it felt that it would be seen to be endorsing it. The Court divided three ways -- four thought there was no Establishment Clause problem with allowing all displays (including religious ones) and singling out them would be discrimination. Three thought a disclaimer or something would avoid the first problem. Two didn't thnk so, especially the flimsy one available.
Over time, I have been somewhat more accepting of a middle path in things involving the First Amendment. Some people online in particular have spoken about the first two amendments in such strident terms that it turns me off. An absolute rule is realistically impossible and practice doesn't recognize it. The result will be various line drawing anyhow. So, e.g., Justice Black said "no law means no law" but somehow managed to be in dissent both in a case involving students wearing black armbands and a jacket with "Fuck the Draft" on it. "Freedom of" includes this.
But, basic concerns should be met and can be done here as well. The endorsement test, however messy along the edges, to me provides a reasonable approach. I do think a holiday display with a single religion dominating would violate such a test. Also, some flexibility here makes sense too as referenced earlier. These cases encourage hypos on each side that suggests both are extreme. In practice, life is a compromise and there is some appropriate middle ground. It honors federalism in the bargain. Inclusiveness should be the name of the game. Holidays are for all of us.
Note: It is sometimes argued that concerns as stated here are in some fashion anti-religious, but this is not my intention though perhaps it might be for some others. Justice Brennan, e.g., was a strong separatist while also repeatedly supporting free exercise more than other justices as well.
Furthermore, the inclusiveness and equality concerns point to a respect for all beliefs. The conservative side arguably is really supportive of religious favoritism or even disrespecting religious belief. Thus, e.g., Justice Scalia argued a cross can be a general memorial for war dead (the lawyer argued many Jews might disagree) and ignored the nuances of various forms of the Ten Commandments in supporting one as a sort of general symbol. But, and this is seen in the history, many religious people are very concerned with details here. The specific wording of prayers could be a matter of great controversy. Religious freedom also does not just mean some general civic religion that tends to have a certain bland Christian flavor.
And, they will disagree among themselves, including let's say as to the legitimacy of usage of a menorah in a public display. Likewise, some bland usage to lessen controversy can be argued as a cheapening of religion. The ultimate dividing line is not lack of respect of belief but a difference of opinion on how to do that.