We suggest waiting until rain is not in the forecast for 24 hours. You'll want to give the nutrients enough time to absorb before the rain hits.
It's okay to wait to apply until the rain subsides, even if your application window has passed. Just be sure to wait at least a week to 10 days in between Sunday applications.
You should not apply Sunday nutrients to a lawn that is waterlogged, or drought/heat stressed. An ideal schedule is to apply a day or two after a moderate watering session or rainfall.