From the London Review, 1863. You wouldn’t believe what the riff-raff are getting up to at St Catherine’s Hill on ‘Tap-Up Sunday’. Four hundred of Guildford’s ‘lowest inhabitants’ were there causing havoc apparently.
The 1898 edition of Brewer’s ‘Dictionary of Phrase and Fable’ points at why:
The Sunday preceding the fair held on the 2nd October, on St. Catherine’s Hill, near Guildford, and so called because any person, with or without a licence, may open a “tap,” or sell beer on the hill for that one day.
Lots more information about the fair (held since the middle ages) can be found in Matthew Alexander’s article on the St Catherine’s Village website.