‘Jam tomorrow’ is some pleasant event in the future, which is never likely to materialize.

‘Jam tomorrow’ originated in 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!’