I’d say about 4-6 weeks before :) 4 weeks is probably ample time! Early July might not see a lot of preschool aged children on holiday - a lot like to be around for things like transition days, leavers parties, graduations etc! What we do is invite friends and then if there’s people declining invite family/my friends kids as back ups if needed x
Some people suggested we host it in June. I’m in Scotland and school holidays start right at the beginning of July. None of my friends have children and I have no family in this country so would hve to invite nursery kids whose parents I don’t know, so sharing same concerns as OP! I was a July baby myself and have awkward memories of parties with many random second and third cousins I never met 🙃
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