One of these days is none of these days

‘I’ll do one of these days.’ This means that you propose to do whatever it is later on. The result is that it is never done at all. Procrastination is the thief of time.

See also:

• Gather ye rosebuds while ye may
• Make hay while the sun shines
• Never put off till tomorrow what may be done today
• Strike while the iron is hot
• Take time by the forelock
• Tomorrow never comes
• What may be done at any time is done at no time