we usually like to have our summer job lined up by December but there are plenty of jobs out there even if you wait until March or April. We had our dream job lined up for this summer before Christmas last year. Came back in April to our old hometown to see the kids, grand kids and take care of our annual physicals before our trek West to our job. They found a small spot of breast cancer on DW's mammogram so we had to miss our May 2nd start date and they gave our jobs to someone else (they were aware of our situation and said they would try to work with us, oh well). DW will be released to travel after her last radiation this coming Friday so we are looking for positions and have several possibilities to choose from.
If you have a specific job or area in mind I would suggest you start as early as you can to get a prime gig. On the other hand of you are flexible you can almost wait until the last minute and still get work, just may not be your first choice of jobs/areas.

Phil