Conditions are favorable for all types of growth in the spring. This means that weed seeds can compete with your grass seed. There will be less competition for your grass if you opt to grow grass in the fall instead.
Too much shade will inhibit grass growth, regardless of time of application. Make sure that your grass seed has enough light and water to properly establish itself. A protective barrier of mulch, such as straw, will help protect your seed from scavenging birds and maintain a more even temperature.
Make sure that you select a proper seed for your region. Grasses that thrive in California and Florida will do poorly in Maine and Wisconsin. Follow recommended application rates as well. Too little will allow weeds to out-compete your grass, while too much will result in poor and spindly growth.