Most homebuyers begin contacting agents in May and June, so it's good to get your home listed in late March on average. That's enough time to make realtors aware of your listing, but not too much time that the home appears to have been languishing by the time peak buying season begins.
However, this rule is a national average, and in colder locales like Boston, the best time to list is in early April, while places farther South call for listings a bit earlier. On average, homes listed in late March sell faster and for 2% more than comparable homes that are listed at other times.