The weather today will most likely be the same as the weather tomorrow
Yesterday’s weather is an Extreme Programming (XP) term to keep teams from over committing during sprints and iterations.
The story goes something like this. Once upon a time there was a government spent a boatload of money on a weather prediction satellite. It took years. Cost of millions of dollars. But they were finally able to launch a satellite that was able to accurately predict the weather about 70% of the time. Not bad.
It was then that someone realized if they simply said today's weather will be the same as yesterday's weather they'd also be accurate 70% of the time.
XP uses this concept to keep teams from over committing during sprints/iterations. It reminds us that a good predictor of the future is what we’ve done in the past.
Read more about Yesterday's Weather in Planning Extreme Programming by Martin Fowler and Kent Beck.