In a legal state
You can be open about it from your first message. Even so, most legal states ban public use: Colorado, for one, rules it out on sidewalks, in parks and in bars. So the relaxed part of the evening usually happens at home, once you both feel comfortable.
In a medical-only state
If you use cannabis on a doctor's advice, share as much or as little as you like, and treat what a date tells you about their health the way you'd want yours treated.
Where it is still illegal
For lots of people, 420 friendly simply means easygoing about cannabis, not looking to break any rules. Keep your dates legal, be respectful of the law and of each other, and let the common ground be your outlook rather than what's in your pocket.