The short answer
A brief squeal on the first few stops of a frosty morning is usually surface rust being scrubbed off the rotors and is harmless. Squealing that continues once the brakes are warm means pads are worn to their wear indicators. Grinding, a pulsing pedal or a pull to one side means metal-on-metal or a sticking caliper and needs attention now.
The harmless morning squeal
Overnight moisture and road salt leave a thin film of rust on cast iron rotors. The first few stops scrape it off, which squeals and can feel slightly grabby. If it clears after a block or two and the brakes are quiet for the rest of the drive, nothing is wrong. Cars parked outside in Winnipeg do this most of the winter.
Squealing that does not stop
Most brake pads have a small metal tab that touches the rotor when the pad is nearly worn out, producing a high-pitched squeal every time you brake, or sometimes constantly while driving. That is the pad telling you it is time. Cold makes the squeal louder and earlier because the pad material hardens. Book an inspection; catching it here means pads only, not pads and rotors.
Grinding: stop guessing
A deep grinding or growling means the pad material is gone and the steel backing plate is cutting into the rotor, or a stone is trapped behind the dust shield. The first is expensive and getting more expensive every stop. The second is a quick fix. Either way, have it looked at the same week.
Pedal pulsing, pulling or a sticking caliper
A pedal that pulses under braking points to rotors that are no longer true, often after hard stops on a hot rotor followed by a cold puddle. A car that pulls to one side, a wheel that is hotter than the others after a drive, or a smell of hot brakes after a short trip means a caliper or slide pin is sticking. Salt and cold make this common on cars that sit for days between drives. A sticking caliper drags, wears one pad out fast and can boil the fluid on a long descent.
Brake fluid and the cold
Brake fluid absorbs moisture over time. In summer that moisture lowers the boiling point; in winter it can corrode calipers and ABS components from the inside. If the fluid is dark or has never been changed, a flush is cheap insurance. Garage 46 measures pads and rotors, checks caliper movement and tests the fluid before quoting, and you see the numbers.