As a rule of thumb, always check out the Monarch Watch website
for information on everything: When do you tag Monarchs? As the
length of daylight shortens in mid August and September,
monarchs in northern latitudes, i.e. near the Canadian border,
begin to migrate. Monarchs farther south will begin their
journey a few weeks later. Tagging and monitoring should begin
in late August in all regions, with a concentrated effort made
in September and early October. A GOOD RULE: when the wild
asters, especially A. novae-angliae, goldenrod and Joe Pye weed
are in bloom, the monarchs are migrating. In much of the lower
midwest, migrating monarchs are attracted in large numbers to a
tall late blooming thistle (Cirsium altissimum) several species
of sunflowers and other species of Asteraceae. Additionally, you
can determine the estimated peak of the migration in your area
based on latitude: Peak Migration Dates