A birthday or wedding brings invitations to help with decorations, transport and planning as well as attend the event. You want to be involved, but several separate requests may compete with the same limited weekend.
Look at the whole set of commitments before accepting another one. Choose the part you can contribute and tell the organiser clearly. If you can attend but cannot help prepare, say that while they still have time to find support.
Avoid leaving a possible yes hanging
When asked about another task, give a definite answer or a specific time for checking your availability. A long series of maybes can leave the organiser treating uncertain help as part of the plan.
If your circumstances change after agreeing, explain the affected task and give notice. Offer a smaller version only if it is useful and realistic. Keep the conversation about what you can contribute to this occasion, rather than turning one unavailable weekend into a judgement about how much you care for the person celebrating.
