I went to the local pool-supply store this weekend to buy chemicals for our hot tub, and, unlike two years ago, it was closed, and only open on weekdays.
I went back on Wednesday and asked why they were closed. The owner replied, “When everybody and their uncle started selling chemicals, it didn’t pay us to be open on weekends anymore.”
Most of their pool- and spa-servicing business is on weekdays. But until recently the store could still cover the variable cost of remaining open on weekends, because customers, most of whom work and couldn’t come by during weekdays, would buy pool supplies on Saturdays. With the competition from Home Depot and others, the store can’t cover the variable costs of remaining open on Saturdays, so it closes. The firm shuts down in the very short run, but it remains open on weekdays, when there is enough business to cover variable costs; and, as my purchases show, they also sell a few chemicals then too.
This would seem to be a fairly common response of small businesses when larger retailers begin to offer a non-specialized competing good.

So you CAN buy pool chemicals on Saturday, despite what the headline says?
Another factor might be if your pool supplier is in a street storefront with only metered, on-street parking that is also used by customers of neighboring businesses, his customers could be having trouble finding a place to park. Always plenty of parking at the Home Depot and it’s free, plus as #1 points out the customer can also pick up his lawn fertilizer, lightbulbs, furnace filters and any other stuff he might need.
Given that the pool/spa business is seasonal, it’s even harder for these stores to manage the variable costs during the winter season. My local shop is closed 3 days of the week during the winter, although Saturday is one of the days they are open; they are however open 6 days per week in season (and they are right down the road from a Home Depot).
You can avoid the inconveniences of being unable to buy your pool chemicals on Saturday by eliminating the need for halogens in your water all together.
Our ionizers (which can be seen at http://www.almostheaven.net/aho/ionizers.htm ) generate copper, silver and zinc ions and completely eliminate the need for chemical halogens such as chlorine. Being able to omit this element of your pool provides not only for a more enjoyable, safer experience for your bathers, but also for the environment.
Anyone who’s tried a pool or hot tub with an ionizers on it will tell you, there’s not other way to sanitize your water.
I prefer to give my business to the small town folks who are just trying to make a living instead of take over the world
I prefer to reward the companies that give me the best value for my dollar. I’d rather not encourage unduly expensive products. It’s the same reason I’ll go out of my way to save a penny/gallon on gas. Sure, a complete fill-up will only save me a quarter, but it rewards the company with the lower price and gives them an incentive to keep or lower their prices.
The store owners may be even smarter than you give them credit for. They still got your business without having to be open on Saturday.
Is that why banks have “bankers” hours too?
Until I had experience with Commerce Bank (Now TD Bank) in the mid 1990s no bank seemed to actually care about servicing customers when the customers could be available for service except on that busy saturday morning from 8am to 12pm (or 2pm.) It looked like ATMs fulfilled that void until online banking and direct deposit cancelled some of the need for consumer level customer focused hours.