Some pleasant event in the future, which is never likely to materialize.
This derives from Lewis Carroll's Through the Looking Glass and What Alice Found There, 1871, in which the White Queen offers Alice 'jam to-morrow':
'I'm sure I'll take you with pleasure!' the Queen said. 'Twopence a week, and jam every other day.'
Alice couldn't help laughing, as she said, 'I don't want you to hire ME - and I don't care for jam.'
'It's very good jam,' said the Queen.
'Well, I don't want any TO-DAY, at any rate.'
'You couldn't have it if you DID want it,' the Queen said. 'The rule is, jam to-morrow and jam yesterday - but never jam to-day.'
'It MUST come sometimes to "jam to-day,"' Alice objected.
'No, it can't,' said the Queen. 'It's jam every OTHER day: to-day isn't any OTHER day, you know.'
'I don't understand you,' said Alice. 'It's dreadfully confusing!'
The phrase caught on quickly and jam tomorrow became a synonym for a 'pie in the sky' promise of good things in the future within a few years of the book's publication. Carroll (Charles Dodgson) would have been aware of the earlier 19th century usage of the word jam - as defined in John C. Hotten's A dictionary of modern slang, cant, and vulgar words, 1859:
"Real jam, a sporting phrase, meaning anything exceptionally good."
The popularity of the Alice books led to the wider use of jam and other phrases were coined in the early 20th century. Anything cushy or rewarding might have been described as 'with jam on it'; for example, this item from Fraser and Gibbons' Soldier and sailor words and phrases, 1925:
"‘You want jam on it’, that is,, You expect too much."
Easy money was called 'money for jam'; for example, The Athenaeum, 1919 - "The great use of jam in the Army ... originated a number of phrases, such as 'money for jam' (money for nothing)."
Socialists often used to ridicule the capitalist system as offering the empty promise of 'Jam tomorrow'. A quotation attributed to the labour politician Tony Benn in 1969 was "Some of the jam we thought was for tomorrow, we've already eaten."
Other phrases first cited in Fraser and Gibbons: