In saves founded with Seasonal closures, from company level 6 (with breakdowns and storms, so a new company, which always starts on 1 January, isn't iced in from its first minute). Real dates, every year:
St. Lawrence Seaway and Welland Canal locks: shut 31 Dec to 22 Mar. Soo Locks: 15 Jan to 25 Mar.
The Great Lakes ports: iced in 15 Jan to 20 Feb, but not in the game's first year (2000). The ice (on the map, and the icebreakers' area) covers the lakes themselves, a circle each (Superior, Michigan, Huron, Georgian Bay, Erie, Ontario), so it never reaches the Atlantic coast.
The Antarctic stations: shut by sea ice 1 Apr to 31 Oct (reachable only in the austral summer).
Nain (Labrador): 1 Dec to 30 Jun. Magadan and Vanino (Sea of Okhotsk): 1 Jan to 15 Apr.
Gulf of Finland ice (within 180 nm of the gulf: St Petersburg, Ust-Luga, Helsinki, Tallinn): 10 Jan to 31 Mar. It stays open, but ships crawl through at 45% speed and 1.5x wear (fuel per mile the same).
A ship won't start a leg it can't finish before a closure: its destination shut when it would arrive, or a canal on the way shut while it's sailing (checked about daily). It waits in port ("Waiting for the Welland Canal to reopen on 22 Mar (its winter shutdown)"), not a hold: no red dot. Ships already at sea finish their leg; ships can always leave a port that's closing.
Warnings in the activity log for closures touching the player's ports (HQ, hubs, where ships are or are bound, their routes) or canals their ships use: 10 calendar days ahead ("The Welland Canal closes for its winter shutdown on 31 Dec: ships won't start legs they can't finish before then."), then when it shuts and reopens, and when the Gulf of Finland ice sets in and clears.
On the World map, while a closure is in force: closed ports have an icy ring, shut canals are frosted over along their path, and the frozen sea is a solid white ice sheet with a soft edge and faint cracks. That covers each closure's ice (the Great Lakes, Hudson Bay, the Labrador and Beaufort seas, the Okhotsk and White seas, the Arctic routes, the Antarctic coast) and every ice zone in season, such as the Baltic's. The sheet lies on sea and lakes, never over land. An icebreaker under way breaks a dark open-water channel through it, edged with broken ice, which freezes over again in about a minute of play. The Northern Sea Route and the Northwest Passage are labeled on the map from mid zoom. A port's popup lists its closure ("Closes for ice from 15 Jan to 25 Mar", or "Closed by ice until 25 Mar"), and the route screen notes each leg's closures. The Tutorial's Seasonal closures tip explains them once they apply.
Icebreakers (see Icebreakers) lead ships through the Great Lakes and Sea of Okhotsk ice and keep the Gulf of Finland's fairways open.
Maritime Empire