In episode 8 of "This is Us", "Pilgrim Rick", Olivia opens up to Kevin and tells him:
how Thanksgiving was always a difficult time for her growing up, with her father always leaving early to spend time with his mistress.
However, Olivia has a very strong English accent, which implies that she grew up in an environment where English accents were the norm - for example, England.
As Thanksgiving isn't a notable day in the UK, is there any in-universe explanation as to why her family were celebrating it?