The World Arc 2016-2017 is showing their Grenada Rendezvous is scheduled for March 17, 2017 and their St. Lucia Rendezvous April 5, 2017 (worldcruising.com). The dates for the World ARC 2017-2018 are March 16, 2018 & April 4 2018 respectively. Skyelark is participating in the 2017-2018 WARC and their charter dates for the Grenada to St. Lucia leg are 3/26/18 - 4/8/18 which looks like a little over a week in Grenada after the rendezvous. So projecting back to the 2016-2017 WARC it might be assumed that many of the participating boats would leave Grenada around March 25th 2017.
Scheduling your charter around that time would give you the benefits of traveling when a fleet of boats are also traveling in the Grenadines (safety in numbers) and also there are families participating so your children would have company along the way.
The downside is many of the anchorages are small and good spots may be hard to come by with the rally going through. There are currently 18 boats participating in the rally.

Drink all day at home, your friends worry about you; do it on vacation and they say "what a good time you're having". Save your friends needless worry, travel more!