Your team is going to miss a committed date. How and when do you tell people?

Tell them as soon as you believe it, not when it becomes undeniable. Bring the revised estimate, the cause, the options (cut scope, move the date, lower the quality bar, add people rarely helps) and your recommendation. Talk to stakeholders directly rather than letting them find it in a status report. Then fix the forecasting process that let the slip go unnoticed for so long.

The moment I believe it, not the week before the deadline. On the last big one I knew at about the forty percent mark, because our burn on the integration work was half what we planned and the trend was flat rather than noisy. I went to the stakeholder with three things: we will be three weeks late at current scope, here is why, and here are the options. One, ship the core flow on the original date and the reporting piece three weeks later. Two, keep full scope and move the date. Three, pull two people off the other project, which I did not recommend because the ramp up cost would eat most of the gain. My recommendation was option one and they took it. The part I care about more is the review afterward: our estimate assumed a vendor API behaved the way the documentation said, and nobody had tested that before we committed. Now anything with an external dependency gets a spike before it goes on a roadmap with a date attached.