The chart that Octazooka answered with is provides insight, but doesn't fully answer the question. (No offense Octa) :P
The 2019 Season's rotation for Standard is any card from the Sun & Moon expansion and after;
Whereas the rotation for Expanded is (as it has been) and card released in the Black & White expansion and after, except for the following (which are banned):
Archeops (Black & White-Noble Victories, 67/101, and Black & White-Dark Explorers, 110/108), Forest of Great Plains (XY-Ancient Origins, 74/98), and *Lysandre's Trump Card (XY-Phantom Forces, 99/119 and 118/119).
So, essentially, you won't be able to use your XY cards in Standard Formatted Tournaments, but you can use them in Expanded Formatted Tournaments, unless they are the aforementioned banned cards.
Source
Hope I Helped!