The satisfaction of raising a bountiful crop of delicious, fully ripe tomatoes can't be beat, and as experienced gardeners will tell you, so much depends upon the weather. Professionals recommend putting out transplants when the outside air temperature is 45 degrees or warmer, usually one to two weeks after your area's average last frost date. Many gardeners use Mother's Day as the start of the season, but it could be earlier, it could be later. If the temps drop after planting your tomatoes, consider wrapping the entire cage in plastic, use wall-of-water plant protectors, or if the starts are small enough, place a plastic gallon-sized milk jug with the bottom cut out over individual plants.
Whenever you choose to plant your tomatoes, one rule of thumb is to plant the starts deep, leaving only two sets of leaves above ground, even if it means stripping off bottom leaves on a particularly leggy start, or actually laying it at an angle with just two sets of leaves above ground. All the little hairs along the stem will become roots, providing that much more water and nourishment to the plant.
Because 'Willamette' is a determinate plant, it has been suggested that it doesn't need support. Just to be on the safe side, please consider putting a cage or stake in when planting, just in case.
Water tomatoes deeply and only when the soil is dry to encourage flower and fruit production. One trick to getting water down to the roots and not on the leaves is to cut the bottom off a 16-oz. to liter-size plastic bottle, throw away the cap, then partially bury the cap end of the bottle into the ground close to the tomato start. Angle it slightly so you can water into the bottle.