The best time to plant watermelon seeds is when the soil temperature is warm, typically at least 15°C to 21°C (60°F to 70°F), after all risk of frost has passed. This usually corresponds to late May or early June in temperate climates. Starting seeds indoors 4-6 weeks before the last frost and then transplanting them outside when the weather and soil are warm can extend the growing season and improve chances of ripening. Watermelons need a long warm growing season of at least 70-85 days to mature well. Key details:

• Sow watermelon seeds indoors mid-April to early May in a heated propagator (18-21°C) to start early.
• Transplant outside only after danger of frost, when soil temperature reaches about 16-21°C (60-70°F), usually late May to June.
• Direct sowing outdoors is optimal when soil is fully warmed, avoiding cold soil that impedes germination.
• Warm, sunny location with well-drained soil is essential.
• Provide consistent water especially during fruit set and growth.

So overall, the recommended time is late spring to early summer, adjusting for local frost dates and soil warmth.