When to plant mint and tomato
Tomato plants can generally go in the ground around mid-May, with a caution that some years the soil is still too cold, so waiting even into June, or using methods to hold heat are required. Here is a link to the OSU Extension gardening calendar for May, with navigations to each month: http://extension.oregonstate.edu/gardening/gardening-calendar/may.
The gardening home page includes a link to download the excellent publication "Growing Your Own", which is valuable because it gives regional advice. This home page also has the "Gardening Tips" search box. Type in "season extenders" for articles about soil temperature and ideas for helping the tomatoes when first planted out in our cold springs. http://extension.oregonstate.edu/gardening/.
In my experience, mint plants can be outside even now, though they grow best next month. Most mint will spread rapidly, so growing in a container placed on a hard surface so the roots won't move through the drain holes into the garden is a safe method. Some gardeners have a separate area they can devote to mint, and as a perennial, you'll always have some once it is there.