Tell them as soon as you know
Send the notice when the slip is likely, not when it is certain. Early news gives the client time to adapt.
Explain in one or two sentences
Give the real reason without drama or blame. Long excuses read as weakness.
Give a new date you trust
Offer a date with some margin. A second missed date does more damage than the first.
Say what is affected
List which deliverables move and which stay on time, so the client can plan around it.
Offer something useful
A partial delivery, a preview or a call can keep trust while the work finishes.
Follow through and review
Send a short update when the new date is met, and note what caused the slip for next time.