IXUS
iShares Core MSCI Total International Stock ETF
stock NASDAQ ETF

\>And I guess if This is right, what’s the day that you need to sell your fund for it to count as a loss.
For the loss sale to not be a wash sale you have to sell it more than thirty days after any previous purchase of a substantially identical security\* and not buy a substantially identical security within 30 days of the loss sale. The previous and subsequent purchase applies across all of your accounts. Your broker will catch it in the same account. They have no way to know about what happened in accounts elsewhere.
As a practical matter the IRS won't know about wash sales across accounts. They only get info about security sales, not purchases. They would only catch it in a deep audit. It is not advisable to do this. I don't play catch-me-if-you-can with the IRS.
The IRS has not clearly defined exactly what substantially identical means. We know from experience that brokers will not flag wash sales of securities with different CUSIPs. You can trade between two S&P500 funds without getting flagged for wash sales. It is probably ok because they have different managers, tracking error, and index sampling that makes them not identical enough. I personally would not do that, because, again, I don't play catch-me-if-you-can with the IRS.
Trading between VTI and ITOT or VXUS and IXUS should be fine even though they are both total market funds. They follow different indexes.
\*The wash sale rules apply to any type of investment, not just securities.

\>And I guess if This is right, what’s the day that you need to sell your fund for it to count as a loss.
For the loss sale to not be a wash sale you have to sell it more than thirty days after any previous purchase of a substantially identical security\* and not buy a substantially identical security within 30 days of the loss sale. The previous and subsequent purchase applies across all of your accounts. Your broker will catch it in the same account. They have no way to know about what happened in accounts elsewhere.
As a practical matter the IRS won't know about wash sales across accounts. They only get info about security sales, not purchases. They would only catch it in a deep audit. It is not advisable to do this. I don't play catch-me-if-you-can with the IRS.
The IRS has not clearly defined exactly what substantially identical means. We know from experience that brokers will not flag wash sales of securities with different CUSIPs. You can trade between two S&P500 funds without getting flagged for wash sales. It is probably ok because they have different managers, tracking error, and index sampling that makes them not identical enough. I personally would not do that, because, again, I don't play catch-me-if-you-can with the IRS.
Trading between VTI and ITOT or VXUS and IXUS should be fine even though they are both total market funds. They follow different indexes.
\*The wash sale rules apply to any type of investment, not just securities.

There are other investment products with different tickers that have tremendously high correlation to VTI and VXUS that you could use partially, temporarily in your tax advantaged accounts as a substitute for a month until the sales in the aftertax account are no longer washes. Interestingly, SCHB is 99% correlated to VTI. IXUS is 99% correlated to VXUS. Buying them for a month or 2 instead of VTI VXUS in the tax advantaged accounts, tax loss harvesting in the aftertax account,then swapping them back into VTI VXUS would have practically no impact to returns / fees whatsoever.

Curious how you guys who do this handle it? I have VTI/VXUS in all of my accounts (401k taxable backdoor Roth and HSA). I’m buying in my HSA 401k and taxable brokerage every 2 weeks. So if I try to sell off VTI in my taxable, I’m triggering a wash sale in my HSA/401k. I don’t really want to pause my 401k/HSA. I was considering swapping my 401k and HSA to VOO and just omitting VXUS and weighting it more in my brokerage. That way I can tax loss harvest without issues with VTI/ITOT and VXUS/IXUS in my taxable brokerage. Is there a better way?