I used to plant everything in the garden the second week in May. Trying to get everything done in May was impossible. So, lately, I have started to plant much earlier. Last weekend I planted radish and spinach seeds. These will be followed by carrots, parsnips, peas, and onion sets by the first of April. These plants will take a hard freeze. Then by mid-April followed by onion plants, cabbage transplants, and lettuce seed. Getting this done before mid-May frees up a lot of time. There are early season extenders to keep your garden warm in the cool spring.
I have used these to jump ahead of the season as an experimental, “fun” project. A lot of times they are more of a hassle but sometimes they really work. You can get floating row covers. These are transparent, lightweight mesh that goes over the row crops. These covers warm the air inside. Some gardeners use these year-round because they keep the pest like grasshoppers and aphids out.
Row tunnels are like the floating row covers but use hoops, so the cover does not rest on the plants. They need to be opened with the temp get too warm.
My mother had some old glass cloches she used. These were solid glass mini greenhouse for one plant as a young tomato plant. You removed them when the day was sunny and warm, so the little plant did not roast. Now people use empty milk jugs.
You have seen the water wells (a round cylinder with tubes, which you filled with water) placed around tomato plants. They used to be clear which allowed mold to grow in the water and now are green or red to reduce this. These really work in that the water absorbs the heat and releases it at night. The water wells may remain in place for the entire season.
I used to take woven fence bent in a 24 in circle 4 foot high placed over the tomato plant and wrapped clear plastic around the fence. There was enough plastic on top to make a lid during cold nights.