The best time to mow your lawn in spring depends on several factors including your location, grass type, soil and weather conditions, and the height of the grass. Generally:

• Mow cool-season grasses (like Kentucky bluegrass, fescues, ryegrass) when soil temperatures reach about 50°F and the grass has grown to about 3 to 4.5 inches. This typically happens between late March and early April in northern regions.
• Mow warm-season grasses (like Bermudagrass, St. Augustine, Zoysiagrass) when soil temperatures reach 60°F or higher and the grass is about 3 to 6 inches tall. This usually falls between early March and mid-May depending on your region.
• Wait until the grass has turned green and is actively growing, not dormant. Avoid mowing soggy soil to prevent damage.
• Follow the one-third rule: only cut about one-third of the grass length at a time to avoid stressing the lawn.
• Ideal mowing conditions are dry grass, low humidity, and mid-morning (8 to 10 a.m.) mowing times.

In summary, start spring mowing when soil and air temperatures consistently warm up, your grass reaches a mowable height (around 3-4.5 inches depending on grass type), and after ensuring good soil conditions—generally from early March to mid-May based on your location and grass type.