Sherlock Season 4: 18 Easter Eggs & References From 'The Lying Detective'
6. Happy Birthday, Sherlock Holmes!
Sherlock receiving a text from Irene makes John realise that it must be Sherlock's birthday.
Sherlockians have long-deduced that Holmes' birthday is January 5th, otherwise known as Twelfth Night. Though it wasn't given in Doyle's stories, fans put together all the evidence and worked out that this must be that exact date.
It's fitting that Sherlock's birthday is mentioned in this episode as, of course, it aired just a few days after that date.