This guide pulls together the 2026-10-03 official activity preview and the 2026-09-25 Producer's Letter #69 to break down the launch cadence, the likely Iberia-style wave structure, operator priorities, and a Sanity budget. Timing comes from published notices; any numbers are ranges based on past events and should be confirmed against the official announcement. Written for Doctors who have cleared the main story and are prepping for the October event. (Guides are also archived in the GameHub guide section.)
Applicable version: current as of the 2026-10-03 update (Arknights has no version-number system; the official notice date is the time anchor)
Arknights "Yesterday's Sea" event info and launch cadence
What the official site has confirmed
The homepage posted "[Activity Preview] SideStory 'Yesterday's Sea' limited-time event opens soon" on 2026-10-03 . Just before that, the studio published an emergency-update notice on 2026-09-29 16:00 and Producer's Letter #69 on 2026-09-25 . Based on the last two years of scheduling, an activity opens roughly 3 to 7 days after the preview drops, so treat early-to-mid October as the window (estimate; confirm with the official notice).
Estimating event length and the shop window
Across the last three SideStories, the main event ran about 14 days with the shop staying open roughly 3 extra days . The two weeks that matter are the front half: clear first-time and fixed stages in week one, then farm material stages in week two. Treat the shop deadline as hard, not soft.
Arknights "Yesterday's Sea" stage mechanics: reading the Iberia wave structure
Why Iberia likely means multi-wave pressure
Iberia is Arknights' long-running coastal region, and past events in it used a two-lane structure of ground push plus air harassment . Expect at least one air-pressure lane with short gaps between waves.
Splitting high ground and ground
Ground units stall melee waves and soak the first push; high-ground units clear air and ranged threats. The steady three-part layout places a high-block Defender at the front, a DPS Guard mid-field, and a Sniper plus AoE Caster on high ground at the back (trade-off reasoning: concentrating limited deploy slots on high-ground DPS that covers both ground and air beats spreading them evenly).
Arknights "Yesterday's Sea" operator priorities
Three roles worth maxing first
Priority order is AoE Caster (clears multi-target and air), high-ground Sniper (single-target burst and long-range picks), then high-block Defender (holding the line). Elite 2 needs chips and Polymerization Agent; keep at least 18 of the mid-to-high tier materials each before the event (estimate; check the official material table).
What you can skip
Pure single-target melee DPS without anti-air is poor value in this predicted wave structure; unless you already have it built, don't raise it just for this event (trade-off reasoning: in multi-wave stages, clear speed beats single-target damage ceilings).
Arknights October Sanity and Orundum budget
| Window | Sanity goes to | Orundum handling | Why |
|---|---|---|---|
| 3 days before | Hold above 1,200 | Claim, don't spend | Leaves room for week-one grinding |
| Week one | First-time and fixed stages | Depends on banners | First-clear rewards are one-time |
| Week two | Material farming | Keep 1,800 buffer | Covers the mid-month second banner |
Sanity: stockpile first, spend later
Filling near the cap before launch is the cheapest way to boost week one. At roughly 240 Sanity of natural regen per day, starting 3 days early frees about 720 extra points for the event.
Orundum and banner choices
October usually brings two banners. To chase both, at 6,000 Orundum per ten-pull you need at least 18,000 banked to call it safe (estimate). If resources are tight, lock one banner and move the rest into operator investment.
Arknights other October content at a glance
Vector Breakthrough #3 and targeted selection
Alongside "Yesterday's Sea", early October adds "Vector Breakthrough #3" and a targeted-selection banner. The selection banner lets you lock three of six six-stars; pick by roster gaps, not by tier lists.
Visual Museum and Integrated Strategies
Late month should bring a "Visual Museum"-style drop and an Integrated Strategies expansion. Both are low-Sanity and work well as pacing between events.
Sanity math for a 14-day window
If the main event runs roughly 14 days and the shop stays open 3 more , a 1,200 opening balance covers about five days of full farming at 240 Sanity per day. Do not dump the whole bank on day one; pace it so week two's material grind still has reserves (trade-off reasoning). Players who also want the Vector Breakthrough #3 rewards should carve out roughly 200 to 300 Sanity before the SideStory opens, since those stages lock when the next cycle starts.
Reading patch dates instead of guessing
Two dates anchor this plan: the 2026-10-03 preview notice and the 2026-09-25 Producer's Letter . Treating the preview as the clock start, an early-to-mid October open is the conservative read. If the official notice later shows a different day, shift the whole schedule by that delta rather than improvising Sanity spends.
What to bank before day one
Keep 18 or more mid-to-high tier elite materials per core operator so you are not farming chips mid-event. Put the AoE Caster and high-ground Sniper on the shortlist first, and treat any pure single-target melee pick as optional. That way the event itself becomes execution rather than a scramble for levels.More information can be found in the Arknights guide.
Arknights "Yesterday's Sea" FAQ
Q: When does Arknights "Yesterday's Sea" start?
A: The official site posted the preview on 2026-10-03; based on history it opens about 3 to 7 days later, likely early-to-mid October.
Q: How much Sanity should I bank before launch?
A: Aim for at least 1,200; at about 240 Sanity of daily regen, starting 3 days early frees roughly 720 extra points.
Q: Which operators should I raise first?
A: Prioritize AoE Casters and high-ground Snipers; keep at least 18 mid-to-high tier elite materials each, and confirm exact counts against the official material table.
Q: Should I pull new operators for the event?
A: If your roster already covers anti-air and multi-target, skip it; to chase both October banners you need at least 18,000 Orundum banked.
Action checklist
- Log in daily and claim dailies so Sanity stays above 1,200.
- Three days out, lock your AoE Caster and high-ground Sniper and bank 18+ materials each.
- Week one: first-time and fixed stages only. Week two: farm materials.
- Treat Orundum as one-banner-only and keep an 1,800 buffer.
- Watch the official notice for the exact open time rather than trusting the window.