Contact landscapers in late winter, January through March, if you want work done in spring. Good crews book out four to eight weeks ahead once the season starts, and by April the spring calendar is often full.
The off-season is the value play. Fall and winter usually mean faster starts, more of the crew's attention, and sometimes better pricing, and fall is a fine time to install most hardscape and plantings anyway. A common rhythm that works well: settle the design over winter, then hold an early slot for spring installation.
Whenever you call, having a clear visual makes the first meeting count. Homeowners who show up with a concept get accurate quotes faster, and OutdoorBrite can turn a photo of your yard into that concept before you pick up the phone.