The depressed day (or days) you have in the 72-96 hours after ingesting ecstasy. (Does not have to actually be a Tuesday) How it came about: Most people who go to school and/or work but who use X will pop a pill on a Friday night and party until Saturday. The imbalances caused by the massive release of serotonin while using the drug causes your receptors to temporarily malfunction and by Tuesday one is usually feeling inexplicably sad or depressed.
I wondered if it was my Blue Tuesday that made sex so awful for me, that evening. Technically it was spot on, but I couldn't get my head in the game and really enjoy it.