Minhag Philadelphia: No Tahanun on Thanksgiving Day

Rating Advisory: B

Last month Charlie Hall left the following comment on Hirhurim:

The 4th of July 1776 was the 17th of Tamuz, 5536. Some congregations in the US don't say tahanun* on July 4.

At the time I doubted this was true and he did not respond to my request for clarification.

While at work today I needed to find some data about the new synagogue that Mikveh Israel of Philadelphia dedicated in 1860, so I began my search at their institutional website. Although I did not find what I was looking for, I did find interesting information, including the special liturgy adopted by Mikveh Israel for shaharit (morning service) on Thanksgiving Day:

• Sing Mizmor Letodah, psalm 100 and the Shira.
• Regular until after the Hazzarah.
• No Tahanunim.* Yehi Amonai Imanu.
• Sefer Torah as regular, New York**: preceded by the prayer for the Government and for the State of Israel. Philadelphia: prayers for the Government and State of Israel follows.
• Sing En Kelohenu and Alenu.
• After the Service, we recite the Hallel*** with festive melodies and without blessings starting from Halelu Et Amonai Kol Goyim.
• New York: Usually the children and their parents watch the Macy's Thanksgiving parade from the steps of the Synagogue on the Central Park West entrance.

It should be noted that Mikveh Israel and Shearith Israel (as well as other American congregations) participated in Days of Thanksgiving and conducted special services already in the colonial era (click here). The composition of such services was determined on an ad hoc basis and the services differed from year to year. When did Mikveh Israel (and Shearith Israel?) adopt the permanent custom to recite Hallel and skip Tahanun on Thanksgiving Day? The literature on the these synagogues does not mention these practices altogether.

Also, does reciting Hallel and skipping Tahanun for an annual day of national thanksgiving in the Diaspora have earlier precedents, or is this a unique minhag (custom of) America Philadelphia? Are there other examples of minhag Philadelphia? (Ok, this is a leading question for another post).

Update: As Steg reported from the holy precincts of Shearith Israel itself this past Thanksgiving Day, it is not just minhag Philadelphia (click here).

Update: For the practice of some Eastern Sephardim to omit Tahanun, click here.

-----------------------
* Tahanun (supplications that focus on forgiveness for our sins) is generally omitted on joyous days, whether marked by annual observances such as festivals or ephemeral celebrations such as circumcisions.
** This is a reference to Mikveh Israel’s sister congregation in New York, Shearith Israel (“The Spanish and Portuguese Synagogue”).
*** Hallel (lit. "praise") is a collection of Pslams recited on fesitvals and other occasions when gratitude to God is expressed.