You could try possibly a few months ahead of time.

However this also can create an issue since the nanny you hire early may get another position which seems more her fit, she may have a change of plans for summer, etc.

I wouldn't think a signed contract by someone would bind you to someone since nanny contracts are not legally enforceable in a court of law.

I recommend doing this as soon as possible. College students may plan out there summers quite early in terms of jobs, internships, travel, and study abroad. A lot of those require applications months in advance. If you want a wider applicant pool, aim to get a post up by the end of the month.

If you are looking to hire a college kid who goes away to school as your summer babysitter you probably should wait until the college kids are back to interview, but post for applications 1-2 weeks prior, closing applications as they start returning.

Assuming you find a responsible person, it'd seem sooner is better than later, so start now.

When should I start interviewing summer babysitters? Assuming the position would be for a college student home for the summer. Thanks!