The promised occasion may change shape before the calendar reaches it.

Plans made far ahead are made by people who do not yet know what the intervening months will bring. The occasion arrives with different weather, different money, different health, and a different set of people free that weekend. Holding it to the exact promised shape can cost more than the thing was ever meant to be worth. What was promised was usually the being together, not the particular arrangement of it.

If something you arranged months ago no longer fits, say so early rather than carrying it grimly. Ask the others which part actually mattered to them, and keep that part.