I have to be in Bangkok, Thailand from Canada (Alberta) on a specific day (ex/ on the 20th). Do I leave Canada on the 18 or 19th?

Airline departure and arrival times are always given in the local time of the place the plane leaves from or arrives in. If a flight says it arrives in Bangkok at 3 pm on the 20th, that is the time and date it arrives in Bangkok, using Bangkok time.

Your friend is an idiot. All published departure and arrival times are in that airport's local time zone. So if the flight says it arrives at 8am on April 20th and the flight is landing in Bangkok, then you will be in Bangkok at 8am local time.

Honestly, I would leave the 18th. So your in a day early, gives you time to relax. My office knows when they book my travel arrangements, I arrive a day or two early. Except for Australia and South Africa, I arrive at least 4 days early. Places like London and Rome, I like to get in the day before and prefer my paperwork, and have a good meal and rest.

Travel Westbound across the dateline, you LOSE a day -

You should be leaving on the 18th to arrive the 20th -

Travel Eastbound across the dateline, you GAIN a day -

If you leave BKK on the 21st, you arrive in Canada... on the 21st -

.

I would leave on the 18th. Once when I flew to Australia from the US, I left on Tuesday and arrived on Thursday. Wednesday was completely lost, kaput.