https://blogs.mercola.com:443/sites/vitalvotes/archive/2026/09/22/why-you-should-boost-your-immune-system-before-fall.aspxWhy Your Immune Prep Should Start in September, Not October Your vitamin D is at the highest point it will reach all year. From here it falls for six months, and the decline starts while the weather still looks like summer. When to Boost Your Immune SystemenCommunityServer 2008.5 SP1 (Debug Build: 31106.3070)