Yes, the dollar bag is only the last day
I'm pretty sure it's just the last day. We always go on the last Monday, which is dime day (10 cents for everything). The website should have a schedule up soon (or already) detailing the prices for each day of the sale.
The $1 (plus tax) per bag price is only during the last day of the sale. For the Spring 2019 Book Sale, the date is Tuesday, May 21, 10 AM to 8 PM.
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