On what date did Basil and Sybil Fawlty marry in the show's backstory?

Basil and Sybil Fawlty married on 17 April 1964.

The date belongs to the fictional history behind Fawlty Towers rather than an event shown directly in an episode. Basil and Sybil are already married when the series begins, and their relationship forms the sitcom’s central comic tension: Sybil is the more capable manager, while Basil constantly tries—and fails—to impose his ideas of class and prestige on the hotel.

The couple also opened Fawlty Towers in 1964, the same year they married. The date is occasionally muddied by Sybil’s joke that they married in 1485, a deliberately absurd reference to the Battle of Bosworth Field. That gag is not an alternative piece of canon. Another useful timeline clue appears in “The Germans,” where Sybil is described as 34, though her exact birth year is inferred rather than firmly stated. The precise wedding date therefore remains 17 April 1964.