When you're considering whether or not it's warm enough to plant out your tomatoes, the blooming of mock orange bushes is a good phenology sign to look for. When you see (and smell!) these beauties, you'll know that the earth has warmed enough to plant out tender vegetables, and that another frost is unlikely.
p.s. - Did you know that the botanical name for tomato, Lycopersicon esculentum, means 'wolf peach'? It's a name that comes from old German werewolf myths!