All Destiny 2 expansions usually unlock with an update on release day. These updates usually coincide with the reset time but not always. Because they unlock with a game update, the release time is worldwide at once instead of staggered like full game releases at midnight. The best way to know for sure when the expansion will unlock is to keep an eye on the Bungie.net website for the update banner (you'll usually see it over the top of the #help forum) or the main TWaB post that will come out this Thursday or next Thursday. You can also keep track of Bungie's Twitter feed. That way, you'll know the exact time once it's fully announced.