Lisianthus bloom groups tell you in what order your Lisianthus will flower. Group 1 flowers first, followed by 2, 3, and 4.
Bloom initiation in Lisianthus is a complicated relationship between day length, light intensity and temperature. If you need to plant in a warmer time of year, choose group 2 or 3 Lisianthus as they won't be triggered to flower until they have developed a good root system.
You can plant all groups at one time without issue. Planting 6-8 weeks before your last frost gives them a long cool period in which to establish roots before the heat of summer. Lisianthus are a perennial in the wild, and this cool period will simulate winter for them. They can take some light frost after they are hardened off.
If your season is long enough, you can expect a second flush of bloom from your Lisianthus. Some even report 2 or 3 additional flushes after the first harvest. We generally just get one cut here in zone 3.
Read up on bloom groups and everything lisi in Bailey's Lisianthus 101 blog post.