Frost is not as much of a concern here as it is in other climates. It is very rare in some of the coastal climates of Southern California. The potential for frost damage increases farther inland, farther north, and at higher elevations. Regardless, it is generally tolerable locally. Even if it is necessary to protect a few marginal plants prior to frost, the ‘average last frost date’ gets little consideration.
The average last frost date designates the end of the frost season for a particular region. Although a specific date, it is an average of dates of the last frost of previous years. It includes minor frost that caused no major damage. Damaging frost, although possible, is unlikely afterward. It becomes more unlikely as the season advances. The process reverses after the average first frost date.
Obviously, average last frost dates are as variable as climates. They are irrelevant for climates without frost. Climates with cooler winters generally have average last frost dates later than those of milder climates. For most of us on the West Coast of California, the average last frost date happens before we are aware of it. Nonetheless, it is helpful to know the date for our particular regions.
Warm season vegetable and bedding plants should be safe in the garden after the average last frost date. Directly sown seed should get all the warmth it needs to germinate. Young plants will not likely experience damaging frost. The weather will continue to get warmer. The days will continue to get longer. Cool season vegetable and bedding plants will relinquish their space as necessary.
Plants that sustained damage from earlier frost can now be pruned and groomed. Damaged foliage that remained in place to insulate inner stems is no longer necessary. Pruning and removal of ruined vegetation stimulates new growth while it will be safe from frost. Aggressively pruning and grooming damaged plants that are already regenerating fragile new growth may be complicated.
Most local climates are beyond their respective average last frost dates. Soon, the others will be too.
Tony Tomeo 
As we just found out here, you cannot rely on the last day for frost.
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Did you need frost for something? The last day is of course an approximation. Frost is technically possible afterward, but merely unlikely.
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I really don’t know our average last frost date but we are aware of when we can safely assume that it has past and that date really relies on the current progress of the season that year and we act appropriately. Usually works, sometimes catches us.
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I do not know mine either, since it is so early. I looked it up this year, and it was later than I thought it was. In Beverly Hills (in Los Angeles County) there is no last frost date according to some publications. Frost does happen there sometimes though.
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Always assumed May 15 but just checked and now the farmer’s almanac says May 2. But I would still hold off until the 15th for tomatoes.
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GADS!
That must by why the Norway maple, which is endemic to climates with such late last frost dates, foliates so late even here. That is one of the main complaints of the tree.
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Growing everything outside here revolves around the dates of the first and last frost. It would be so good to have a longer growing season.
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Perhaps, but the disadvantage of a longer growing season is insufficient chill for plants than need it, like many of the cultivars of apple.
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True, it does limit things that you can grow.
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