When the night time temps. are consistently above 50 degrees.

After you have mowed real grass, not just weeds, for the second time. That way you can be sure the roots are awake and ready for fertilizer.

Note that this is only for bermuda. Other grasses do not need fertilizer early in the spring. For that matter, neither does bermuda.

I see you also asked how often on another post. Bermuda can take a high dose of high N fertilizer once a month or every 6 weeks throughout the growing season. If you are going to fertilize it that often I would make at least one application per year of organic fertilizer. By using so much chemical fertilizer the soil can "use up" the microbes in the soil. Organic fertilizer feeds them. There are people having good success using only organics (alfalfa pellets at 20 pounds per 1,000 square feet) every 6 weeks. It is not nearly as high N as chemical, but with frequent apps, it works well.