Update [Mon 17th Apr, 2023 17:15 BST]: If you didn't have a chance to stock up on Tera Shards and EXP Candy, now you've got another chance, as the Blissey Tera Raid event is returning this weekend.
Alongside a 7-Star Typhlosion, Blissey will be more likely to appear in 5-Star Raids and it will drop large quantities of Tera Shards and EXP Candies from 21st to 23rd April.
Original article [Mon 20th Mar, 2023 10:15 GMT]: Pokémon Scarlet & Violet's next Tera Raid Battle Event has been announced, and it's coming very soon — this weekend, even! From 24th March to 26th March, Blissey will be the next Raid Event Pokémon (via Serebii).
This Blissey is a bit different from the other standard Raid Event Pokémon. It doesn't have a particular Tera type; it can come with various different types and gives out increased EXP Candy and Tera Shards if you manage to beat it.
That means this Raid will be excellent for levelling up your Pokémon or if you're looking to change your 'mon's Tera Type.
Blissey is already a popular Pokémon in normal Tera Raids as they have a higher chance of dropping Herba Mystica, but as Serebii has noted on Twitter, Event Raid Pokémon don't give out Herba Mystica, so it's unlikely this Blissey will also give these out. EXP Candy and Tera Shards are pretty good though.
This won't be the only Tera Raid Battle Event taking place this weekend. The second wave of 7-Star Greninja Raids kicks off at the same time on 24th March until 26th March. It's a raid bonanza, then.