Apple has started sending emails to registered iOS developers today inviting them to participate in a WatchKit lab held at the company’s Sunnyvale, CA campus. The lab will run from March 16th until April 30th, so developers will still have the ability to take advantage of the offer even after the device has shipped.

The lab will give developers a chance to test their software on the actual device instead of the simulator built into Xcode. This will be especially important for those who want to submit their apps for day-one availability, since Apple has instituted a rule that all watch app submission must include a screenshot created on the device itself, not in the simulator.

Each developer will be allowed to bring one additional team member to the lab, Apple is recommending that each development team that participates send both a software engineer and a designer. Developers will need to sign up quickly, as space is limited. Teams who take part in the lab will be able to reserve one day to test.

The full text of the email is below:

The lab in Sunnyvale, California runs from March 16 through April 30, 2015. You may bring one other member of your development team in the iOS Developer Program. We recommend that an engineer and a designer attend from each team.

Seats are limited. Sign up today.