Frost in my garden can arrive as soon as a month from now. Six weeks from now it's likely. In two months I can expect the nights to drop below freezing with regularity. While many serious gardeners can recite their last frost date, very few can do more than guess their first frost date.
The first frost date is the average date, based on years of local climate data, when you can expect the temperature to drop below freezing for the first time. A good place to start in finding yours is to go to the National Climatic Data Center and NOAA website. Your local Extension office or website should have specific information about your neighborhood (here is Colorado's); it often varies slightly from NOAA's information.
Like the last frost date in the spring, your first frost date is officially three different dates. When you look up your frost frost date you'll find one each for 10 percent, 50 percent, and 90 percent probability. On the first first frost date there is a 10 percent historical record of freezing temperatures hitting before that date. The second first frost date is also called the average first frost date; 50 percent of freezing occurrences happened before that date and 50 percent after. The final first frost date records the point when 90 percent of historical frosts have already occurred. How you use these dates varies depending on what and how you're gardening.
For warm season plants I use first frost date in a similar manner as I use last frost date. In the spring I use the 10 percent date to determine planting and sowing. That's the calendar date after which only 10 percent of frosts have occurred; in other words, the weather is getting warmer and there is decreasing likelihood that freezing weather will occur after that point. In the fall for the 10 percent first frost date, it's getting colder but historically only 10 percent of freezing nights have occurred before that date. There is increasing likelihood of freezing temperatures after that point.
The 10 percent first frost date for Colorado Springs is 20 September (according to CSU Extension it's 18 September). That's when I can expect my tomatoes, peppers, beans, and other warm season plants to begin the slow march toward an unwelcome demise. It's not written in stone, but it's a target for me to begin looking closely at weather forecasts. That's the time when I should have my hoop houses and mini greenhouses back in place to help protect the plants from occasional low temperatures.
For cool season plants the 10 percent date presents little concern. An occasional frost rarely affects them adversely and can actually improve the flavor of many. I also use that date as a target to have my fall plants in the ground. The soil will still be warm enough for root crops, bulbs, trees, and many cool season seeds to develop.
Two weeks later on 3 October the historical 50 percent first frost date hits (6 October for Extension). That's the average first frost date and is the date that many catalogs and gardening manuals are referencing when they say to plant fall plants at a certain point before the first frost. After that time the soil can be expected to be below effective germination temperatures and unprotected warm season plants are almost certainly dead. The tender cool season plants are more likely to see damage after this date.
By my 90 percent first frost date, 15 October (18 October for Extension), the garden is cold whether an official frost has hit or not. It's still a few weeks from the first expected snow, but even with garden extenders like the mini greenhouses warm season plants will probably be too chilled to continue producing. Cool season plants should still be okay and can be expected to continue production.
For my planning purposes I look at the end of September and beginning of October as the time to put most of my gardens to bed for the winter. I want to have fall bulbs, including garlic, planted by then. Many gardening activities like watering and weeding decrease dramatically. Others like removing dead and dying plants increase.
Your first frost date is almost like a negative prognosis from the doctor. You get bad news with a percentage attached. Gardening concerns are not nearly as critical as negative health issues, but similarly, knowing about them allows you to plan for them. Prepare yourself by knowing your first frost date so you're not caught by surprise when an unexpected frost devastates your prize tomatoes. Use it to plan your fall gardening activities. Move to another level of gardening by being better informed.