The best time to plant butternut squash seeds outdoors is after the danger of frost has passed and soil temperatures have warmed to at least 60°F (15°C). For most regions in the United States, this will be from late spring to early summer. If you're starting seeds indoors, begin 4-6 weeks before your region's last expected frost date. Adjust planting times based on your local climate and the length of your growing season to ensure your squash has enough time to mature before the first frost.