Calibration for race day

What is the best way to ensure a calibrated power meter?

The most important things are:
1. The pedals are at thermal equilibrium, i.e. they've been sitting in a more or less constant temperature environment for at least 15 minutes.
2. You have done several hard efforts, such as 15 to 20 second sprints at 150% FTP or more, since you have installed the pedals. This is significant if you traveled to an event and had to remove the pedals from the cranks to get the bike into a travel case.
3. The spindles are torqued tightly into the cranks before you tried #2.

Should I calibrate the day before when I check-in the bike? Or in the morning of the race (6 am)?

It really makes no difference, so long as you observe #1 above. Do not pull your bike out of a heated indoor space and calibrate five minutes later in near-freezing temperatures.

I am under the assumption that we calibrate before each ride to compensate for temperature changes - is that correct? If so, would it make sense I calibrate the day before in similar temperature conditions?

The main purpose for calibration is to track changes in gauge offset due to pedal installation variations, and secondarily to set a reasonably close reference for the current temperature state. The pedals are fully temperature compensated and as long as you observe the important points above, you can ride through 20 to 30 degree C temperature swings without loss of accuracy.

Edit: If you clip your shoes into the pedals as part of your T1 setup, then make sure you disable auto-calibration through the head unit settings.

Many thanks! Helps a lot :)