During the entire series, holidays - Christmas (Charlotte put up a tree in July), Thanksgiving, Valentine's, Halloween, Fourth of July, etc. - are never celebrated with the exception of Sabbath in Season 6, Episode 4. Birthdays are also not celebrated with the exception of Season 1 Episode 1 (Miranda), Season 4 Episode 1 (Carrie's) and Season 6 Episode 12 (Brady's).
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