Meaning: Some pleasant event in the future, which is never likely to materialize.
Origin: 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!'