Bag days are almost always scheduled for the last day of a multi-day sale, when the Friends group would rather move inventory than box it back up. Because the price is per bag rather than per book, the math flips: your goal shifts from picking a handful of great titles to packing volume efficiently while still keeping quality high.
In practice this means arriving with a plan. Scan or sort in the aisles, build a staging pile, then load your bag last so nothing marginal takes up space. Many sales cap bag size or require you to use their bags, so check the rules before you show up with reusable totes or banker's boxes.
Pricing impact is significant. A hardcover that cost $3 on opening day may effectively cost 30 cents on bag day, which is what makes the format popular with resellers, teachers, and Little Free Library stewards. The trade-off is that the best-condition and highest-demand titles are usually gone by then.