Now is when you should treat your lawn to remedy soil nutrient inefficiencies. Get a soil test for your lawn first. This will tell you where you stand instead of blindly applying unnecessary fertilizers.
The pH is off very often and affects how well plants uptake nutrients in the soil. The wrong pH could cause a lockout of vital nutrition and leave you scratching your head as to why your plants are worsening.
Use sulfur amended fertilizer products to bring your soil pH down. Use lime amended fertilizer to bring your pH up. It is a lot quicker and easier to bring your soil pH up than it is to bring it down. Keep that in mind when altering it and be sure to read your soil test before starting this process.