Given the home repair nature of Home Depot, weekend downtime can be more costly than a Wednesday or Thursday. As for the time, one Home Depot person said it was simply that mornings tend to be more revenue-intense on the retailer's site than any other time of day. Therefore, the word was that mornings need to be preserved. Given that the upgrade was supposed to take 18 hours, the only way to try and preserve morning uptime was to start the shutdown at noon and hope that 18 hours didn't grow.
Some E-Commerce folk still grumbled that sites today—with mirroring and cloud options—shouldn't have to shut down at all for simple scheduled software upgrades.
Although there certainly is a lot to that argument, the cost and disruption of making those arrangements—plus the potential for lost or perhaps doubled transactions when the duplicate site is put up and then again when it is taken down—might have been seen as risky on its own. And given that it was indeed limited to 18 hours and brought relatively minimal risk, there's an argument that Home Depot's move was probably shrewd.
Should a $68-billion chain have to shut down for simple software upgrades? Probably not, but nor should it have to deploy an extensive system just to avoid it. Sometimes a shutdown to avoid issues shows a lot of wisdom.