Original speculation post is below, but the official announcement came out today (July 15) here:
https://elderscrollsonline.com/en-us/news/post/58487, and also the official discussion thread here:
https://forums.elderscrollsonline.com/en/discussion/537573/official-discussion-thread-for-defend-the-home-of-the-high-elves-during-the-summerset-celebration.
Updates to original post included below now:
This thread is just to generate a bit of hype and awareness about the next upcoming event.
So far, the only sources we have are the PTS patch notes v6.0.2 and the July 2020 Crown Store showcase article (which mainly says "to be announced"). Refer to the actual ZoS announcement link above for final details.
What we know:
Summerset Celebration Event
Return to the Summerset Isles! Earn bonus rewards by completing daily quests in Summerset, exploring the zone, or battling powerful monsters.
- Each daily quest you complete in Summerset, as well as completing the weekly Cloudrest quest, earns you a Summerset Coffer which contains crafting materials, style items, overland set pieces, flavored salable items, and chances at valuable Summerset motif and outfit style pages!
- In addition, the very first of these quests that you complete each day (per account) instead awards a Glorious Summerset Coffer (which is even more bountiful than the normal coffer), along with 3 Event Tickets.
- Continuing to explore Summerset after your daily quests are done is encouraged! Additional non-Glorious Summerset Coffers have a good chance to drop from Delve Bosses, World Bosses, and Geysers; a lesser chance to drop from Safeboxes, Thieves Troves, Treasure Chests, and Psijic Portals (but only the ones in Summerset) and a tiny chance to drop from crafting nodes, regular monsters, or random lootable containers such as barrels, crates, and sacks.
- All coffers have a chance to drop a style page for this event’s new outfit style, the Snowhawk Mage.
- Furthermore, the Cloudrest weekly quest grants two of its normal reward boxes instead of one, and defeating the Trial in hard-mode grants you two Crests of the Welkynar instead of one.
- If that wasn’t enough, during the celebration all Summerset resource nodes, creature crafting drop, daily quest reward boxes, and Psijic portal loot are doubled, and World/Delve bosses give you two draws from their usual loot tables!
The Crown Store Showcase also indicates that Psijic Crates, the Sagacious Psijic Senche mount, Enigmas of the Elder Way Music Box, and the "baby Raz" pet will be available for purchase during the event, as well as additional event tickets, should you have crowns/cash to burn.
Update: as mentioned below by
@Taleof2Cities, the impressario will have a new Mossheart indrik mount berry on sale during the event. (Apparently two more Indrik skins have been datamined for the third and fourth quarters too, the Red/crimson and Cursed Indriks. Lots of Indrik fatigue here though.)
Update: as mentioned below by
@AmaltheaWinter, the impressario will have Alinor-themed furnishings on sale for event tickets during the event. The official announcement also lists all furnishings available, including a painting that you can also get via one of the zone quests. Most of the event furnishings are craftable and may also be available for purchase on guild traders, sometimes inexpensively. Most of the event ticket furnishings are craftable (prices ranging from 4-25k depending on the item), but three are not, i.e. the "
Alinor Monument, Marble", "
Alinor wall mirror, Ornate", and the "
Alinor Pergola, Blue Wisteria Peaked". Furnishings bought with event tickets are usually bind on pickup, and not tradeable to other players; they might just be something nice to get if you are low on gold but have event tickets to spare.
Update: The "Psijic Lighting Globe, Small" furnishing (on sale for 1 event ticket) is craftable - furnishing plan is bought from the merchant in Arteum - but always bound, so not tradeable, you have to make it yourself.
It's a double drops event (excluding the usual, i.e. Fishing, water resources, treasure chests, butterflies/torchbugs etc.), with 3 tickets per day, and with a new special outfit style available from special daily bonus reward coffers, kinda like in the Murkmire event. (That is, the special Summerset coffers have a low drop rate EXCEPT guaranteed Glorious coffer from the first daily repeatable quest of each day OR the Cloudrest weekly reward urn.) Bosses may have a higher drop rate for reward urns than other sources, but remember that only 12 people can get loot from any boss kill, and there is normally a 5 minute cooldown on getting loot from kills of the same named (delve) boss. I don't recall if Geyser rewards are also limited to 12 people or are unlimited like dolmens or other analogues in other zones.
Update: It's also a double-berry event, i.e. twice as many tickets as usual (counts as two of the four events for the quarter), lasting 13 "days" (July 23 to early on August 24). (I know that's not exactly 13 days, but for those hunting event tickets, that's the number of special "Glorious Summerset Coffers" that you can get. Three event tickets per day, for a total of 39 achievable.
What is likely but we DON'T know (yet):
- Wrong: The Summerset Celebration is likely to be a week-long event, starting on Thursday, July 30. Most events start on a Thursday, and most events give just over 20 event tickets. The Queen's-Eye Spymaster costume will be on sale from July 30 to August 4, giving a hint as to the possible event dates, although an earlier start on July 23 is certainly possible, as is a 10-day duration.
- Still unknown: The Grand Psijiic Villa (and possibly Colossal Aldmeri Grotto?) will likely also be on sale in the Crown store.
- Confirmed: The Summerset DLC will be on sale at a reduced price during the Celebration. (50% off during the entire event.)
- Negative: It's unknown if[ZoS update:] Giant Clams and/or Geysers will NOT give double drops. I really hope so, for the chance of more rare Clam Gall and Powdered Mother of Pearl alchemy material drops.
- Negative: It is also unknown if bonus rewards will drop from fishing rewards. [ZoS: They won't]. There is one lead that can drop from fishing in Arteum though, so I might do some fishing anyway. I'm also hoping for a small chance of finding a Summerset Coffer while fishing, although the ZoS response to that was unclear and might be negative.
Note that you will want to try farm as many Summerset coffers as possible (assuming the drop rate is not as terrible as during the Murkmire event), for a chance of Clam Gall and Powdered Mother of Pearl alchemy material drops, Culanda Lacquer style materials, [possible motif pages?], new style pages, and Alinor furnishing plans.