Is it 1888, 1889 or 1890?
In 1.02 Violet says that Robert married Cora against her wishes "24 years ago". As the episode is in 1912 this would make the marriage 1888.
The press pack says 1889.
Episode 5.01 says 1890 due to "34th anniversary".
We know it was either February or August that Cora and Robert married; Mrs Hughes says their anniversary is a "Saturday". Saturday the 16th fell in only February or August 1924. This means that Violet's word in 1.02 are accurate -- the family are in half mourning and Sybil says the family can be in half mourning by SEPTEMBER meaning both dates for the anniversary have passed.