Canadian worker travelling to the US–can I work for 1-2 days for my Canadian employer?

De jure there are dozens of complex laws and regulations regarding remote employment, under which you may or may not need a special visa in order to work in a given country. Tax laws are an additional complication, where countries such as the UK can deem you as a tax resident for spending as little as 16 days on British soil.

De facto, as long as you don’t mention your remote job to immigration personnel at the airport, there’s a 99.99% chance no one will ever find out. There are millions of people in the US breaking the law by being employed at on-site jobs without a proper work authorization, so digital nomads are a very low priority for law enforcement. Same goes for taxes – the IRS has enough local tax evaders to deal with before they’ll start worrying about Canadian tourists doing some work remotely for their domestic employer.

There may be differing opinions on this, although I believe that it is perfectly OK since you are not earning income from a US source.

What is absolutely certain is that literally millions of people do this every year and none have problems. As long as you are doing exactly the same thing as you would be doing in Canada, nobody will care. This assumes that your reason for the US visit is entirely separate from your work.

You’ve asked for "incontrovertible source", so that’s what I’m providing. As you can see from the other answers, a lot of people feel comfortable with ignoring the law excusing themselves one way or another, or just plainly claiming "who’s going to know", but that’s a risk that only you can decide whether you’re willing to take. You’d be breaking the law.

From what I’ve gathered, this is fine because I’m not working for a US employer and my stay is de minimis (for income tax purposes–I’ll be in the US fewer than 183 days). On the other hand, I’ve read opinions that say simply ‘no, this can’t be done.’

Taxes

From tax perspective, the short stay means it would be disregarded (as long as you stay less than 183 days and earn less than $10,000). That would be due to the US-Canadian tax treaty Article XV, without it you’d owe taxes on the US-earned income from day 1 (after you’ve exceeded 90 days in the US or once you’ve earned more than $3000). Any income earned while in the US is US-sourced income and is taxable in the US, but for employees of foreign employers there are specific rules that allow avoiding US taxation under very limited circumstances (see pub. 519). The tax treaties supersede these rules and are generally much more favorable. Per the IRS:

A nonresident alien (NRA) usually is subject to U.S. income tax only on U.S. source income. The general rules for determining U.S. source income that apply to most nonresident aliens are shown below:

See Chapter Two of Publication 519, U.S. Tax Guide for Aliens for additional details.

Item of Income                      | Factor Determining Source
------------------------------------|----------------------------
Salaries, wages, other compensation | Where services performed

Immigration

From immigration perspective – no, this can’t be done. You’re not allowed to work in the US on a B1/B2 visa. You are allowed to conduct business meetings, participate in conference, or collaborations, etc. The full list of permissible activities when in B1 status is here (note: you have to enter in B1 status for any of it to be relevant, not B2). That list already includes references to the "Matter of Hira" case.

The "usual work, just from Air BnB" – no, not allowed.

From the US Department of State:

An individual on a visitor visa (B1/B2) is not permitted to accept employment or work in the United States

You’ll see some folks on this forum trying to claim that the fact that the employer is foreign or that the work is coincidental to a vacation matter, but strictly legally speaking they don’t.

You’ll also see some folks on this forum fiercely advocating that the Matter of Hira precedent nullifies any DoS or USCIS guidance. They’re wrong. As shown here, there are only very specific business activities allowed under that precedent. There are potential cases where employee may be allowed to work in the US on a B1 visa, but that has to be a special "BILOH" (B1 in lieu of H1B) visa, annotated as such. The DoS is proposing to eliminate that entirely.

You can test any of that by asking the CBP officer at the point of entry (but be prepared to be turned around).

If you know that you shouldn’t ask the CBP officer this question – then you know the answer.