Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to congratulate someone on their wedding, use this factual base: name the milestone and, when you know it, the effort or meaning behind it.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer warm recognition and let the recipient decide whether they want a call, visit, gift, or quiet celebration. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not make assumptions about pregnancy, money, relationships, health, or what the achievement should mean next. Follow the person's lead on privacy before sharing their news or photos with anyone else. You do not need the other person to agree with a boundary for you to follow it consistently.