@JeremyECrawford If you're invisible and you Ready a spell, do you become visible, ruining your invisibility spell?
— J. Richardson (@jsrichardson23) September 14, 2015
If you're under the effect of the invisibility spell, readying another spell does end the invisibility. #DnD https://t.co/zhgziRAqN7
— Jeremy Crawford (@JeremyECrawford) September 16, 2015
Shouldn’t it end when the spell is cast, not readied? What if the trigger never occurs? You can’t apply results before the action happens.
Readying a spell is when the spell is cast, not when it is released with your Reaction (before your next turn). As noted for readying a spell, you cast it normally and then hold the energy of the spell for a few seconds until it triggers; if your turn comes back around again (x seconds later) before it’s triggered then the spell dissipates and is lost. That’s part of the risk of readying a spell and why in most cases a caster will ready a cantrip because there’s no real loss in that regard.
The Invisibility is also lost at the time of readying for another reason: Invisibility (spell) is a concentration spell, and readying a spell requires your concentration to maintain/hold the energy until released within the following several seconds — once you begin readying a spell, Invisibility drops regardless.