Spaghetti squash need a long growing season and warm temperatures, and that includes warmth at planting time. So any seeds would have to go out after your frost date as passed. But starting your seedlings indoors is a common way for gardeners to start their squash plants. Plant your seeds inside about a month before your local last frost date. Squash seedlings will grow fairly large in a month, so small 6-pack style seed trays won’t likely suffice. If you start them that way, you’ll have to replant them in larger containers before your outside plant date. Three or four seeds can go in each 3″ pot, about 1″ deep under the soil. They’ll need to be kept somewhere warm as well as sunny in order to properly germinate and sprout.
Source: Backyard Gardening