When is a good time to go visiting potential post-doc opportunities?
The answer depends on your geographical and academic areas. 


Postdoctoral positions in Europe are often attached to specific research groups and are funded through grants obtained by the head of laboratory/research group. As such their availability are often announced after the successful grant application, which means sometimes in the spring in general. On the other hand, if you have a specific person you want to work with and you have discussed this, it is good to get started early so you can help out as much as reasonable with the grant application process. (This is assuming that you have struck a mutual agreement that she will hire you if the grant is successful.)
Postdoctoral positions in the United States and Canada are, depending on field, are sometimes funded as in the European case described above, and sometimes funded by the department (departmental, "named" postdoctoral positions). In the latter case the application process may have begun already and you should consult the appropriate trade magazines or bulletin boards to keep yourself posted of these opportunities. (I see that your profile indicates an applied math background; many math positions are posted on MathJobs and AcademicJobsOnline, and you should of course keep an eye on SIAM publications.)

In fact, the schedule is also similar for faculty positions in the United States: you generally expect to be applying during the fall and winter (and sometimes the summer) of the year prior to the start date. (Something you should keep in mind after you land your postdoc job.)
The various countries' equivalents of the National Science Foundation also offer postdoc funding opportunities with varying deadlines. For example, the United State NSF funds postdoctoral research in various fields. And you are likely already too late: the Mathematics deadline was last week. While the Swiss NSF has a postdoct program whose due date is not under February next year.