From the U of MN website:
Late summer and fall are the principal times of year to fertilize lawns (Table 6). This is contrary to the traditional springtime application. Some advantages of fall rather than early spring applications are shown in Table 9. Early spring applications of nitrogen cause a surge of top growth in the plants, which makes the lawn look nice in the spring but depletes the plants� energy reserves. Consequently, when summer stress periods occur, plants are weaker and less able to survive. Applying fertilizer in late August or early September will provide the plant with adequate nutrition to overcome any summer stresses. In addition, an application of fertilizer in late October or early November, when top growth is minimal but when soil temperatures are still warm enough for nitrogen absorption, plants resume growth and green-up early the following spring without the excessive shoot growth associated with early spring nitrogen applications.

Table 9. Benefits from applying lawn fertilizers in fall rather than early spring

Lengthened period of green in the fall
Earlier green-up in the spring without stimulating excessive shoot growth
Carbohydrate reserves (a measure of the energy
stored in the plant and available for growth) remain higher during the spring and summer period
Reduced incidence of summer diseases

Environmental Conditions
When fertilizing lawns, particularly with quick-release nutrients, it is important to consider the weather and turfgrass conditions to achieve maximum effectiveness of the applied fertilizer. Ideal conditions include a cool day with a good rainfall or watering immediately following the fertilizer application to wash the fertilizer off the leaves and into the soil. As the temperature increases, the potential for damaging the leaves through fertilizer burn increases; consequently, care must be taken by applying a lower rate of fertilizer, using slow-release forms, or washing fertilizer off the leaves immediately after application. Unless absolutely essential, fertilizer application using quickrelease nutrients should be avoided when air temperatures are higher than 85�F.