Keep in mind when planting that Guelph Millennium is thought of as hardy, so it can be safe to leave outdoors for the majority of winter (although if in doubt, using a row cover is often a good idea).
Soil temperature should be kept higher than
21°C / 70°F to ensure good germination.
By our calculations, you should look at sowing Guelph Millennium about 90 days before your last frost date.
As Guelph Millennium is hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.