Yes Rainy Days do affect a WiFi signals strength.
But if you're inside your house the rain should have no affect on the signal strength to your WiFi signal and your WiFi should not be affected by the rain or cloudy days while inside the house.
But if you take your device outside your house to use the WiFi then yes it can affect the signal strength of your WiFi since that's what I experience as well when I go outside to sit on my covered patio during the rain.
When I do that my signal drops only when it's raining but is not affected during sunny days.