Temperature
Here's the Cheat Sheet:
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Soil temperature decides planting time, not air temperature or the calendar. Use a soil thermometer.
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Start tomatoes indoors 6 to 8 weeks before last predicted frost. Peppers and eggplant, 8 to 10 weeks. Frost is not gentle, and it does not negotiate.
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Harden off seedlings for a minimum of 1 to 2 weeks before transplanting.
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Blossom drop above ~35°C is a heat problem, not a fertilizer problem. Don't feed a heat-stressed plant.
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Soil temperature, not air temperature, decides when you plant seeds. A warm sunny day means nothing if the soil underneath is still cold. Seeds sitting in cold soil don't germinate. They rot. Get a cheap soil thermometer. It will save you more grief than any calendar date ever will.
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Cool season crops, lettuce, spinach, peas, kale, actually like it chilly but don't push it. Wait until the forsythia and dandelions are blooming, or until soil is about 10oC.
Planting Out Seedlings:
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Phenology has cues for transplanting out. Tomatoes go out when lily of the valley is in full bloom. Peppers and eggplant wait for bearded iris or daylilies to start blooming, a bit later than tomatoes, since they're fussier about cold. And old farmers will wait until after the full moon in May or June, (depending on your zone) as nights are often clear and cold during the full moon.
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A warm afternoon is not the same as a warm season. One mild day does not mean the soil, or the nights, are ready. It is ESSENTIAL to harden off your seedlings first, a week or two. Choose a shaded sheltered spot at first then gradually move them into stronger sunlight and wind. Skip this step and watch weeks of indoor coddling undo itself in one afternoon.
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Blossom drop is a heat problem, not a fertilizer problem. Once daytime temps climb into the mid thirties C, plants abort their own flowers rather than waste energy on fruit they can't support. Feeding a heat stressed plant more nutrients doesn't fix this. It just wastes fertilizer on a plant that has already checked out for the day.
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Nighttime temperature matters just as much as daytime. Plants need a cool down to recover. A string of hot nights stresses them as much as hot days, sometimes more, and unlike you, they can't just turn on a fan and complain about it.
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FROST. Frost settles in low lying areas and we all get caught off guard. If the expected LOW temperature is around 3oC cover your plants or bring them in. If you are caught off guard, one way to help your plants recover is to water before the sun rises. A gentle watering to wash the frost off the leaves before it penetrates can save tender plants. If it's a hard frost and leaves are frozen solid they will not recover. Cut the leaves and hope there are enough food reserves in the roots to re-establish growth.