There's a huge fare war to Amsterdam (AMS) going on at the moment. It's extremely rare to see the major airlines drop a price to Europe like this from Halifax.

Update: Looks like US Airways has bumped them up to $390 now.

UPDATE 10:13PM: Flights to Los Angeles and San Francisco now look to be down to $316 to $340 roundtrip after taxes on US Airways on certain dates.

Looks like Continental has drastically dropped the price of a last minute flight from Halifax to Newark (EWR) this upcoming weekend.

Yes, there are some more of those cheap last minute flights to Chicago on United (direct each way). They're about 50% cheaper than the cheapest flights to Chicago normally cost from Halifax.

I'm not really sure why United does this, but every now and then they offer an extremely cheap flight to Washington, DC for the upcoming weekend.

Update @ 12 AM: The promo code (which expired at midnight) made this deal a bit better, but the real key here was the large drop on the base fare. Looks like it works out to $383 roundtrip after tax without the promo code, which is still a pretty amazing price for Halifax to Calgary, especially for summer. And as of this writing, still available.
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Update: Westjet looks to have just matched the Air Canada sale. Today looks to be an amazing day for inter-Canada travel prices.
Westjet doesn't seem to have an official page up yet about their sale, they're probably scrambling to get their prices changed to match Air Canada.
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The cheapest flights to Washington, DC from Halifax usually run around $600 roundtrip after tax these days.