Blizzard have posted a list of weekly activities and rewards that will not be available until the very first reset of the expansion, 1-2 days after launch.

DF First Days Content (Source)

With the timing of the unlock of the Dragon Isles (Monday, November 28 at 3:00 p.m. PST / 5:00 p.m. CST / 6:00 p.m. EST in this region), players will find themselves leveling up with less than a day to go before the weekly reset.

We’ve configured Dragonflight to prevent most weekly rewards from appearing prior to that first weekly reset. As a result, the following content will be unavailable until the new week starts:

Mythic Dungeons

Dragonbane Strongbox from Siege of Dragonbane Keep (Epic quality)

Grand Hunt Spoils (Uncommon, Rare, and Epic quality)

The weekly quest “Community Feast”

The “Aiding the Accord” weekly quests

The “Show Yer Mettle” weekly quest

Weekly Valdrakken professions quests of any kind

Weekly PvP quests rewarding honor

We want players to level at their own pace without feeling like they’re missing out on a week of rewards if they fail to reach level 70 within a few hours after the expansion launch.