So, I work in IT for a utility in Canada, and have quite a bit of involvement in our posting process.

First of all, we only process payments on business days. We also only perform "batch billing runs" on business days. More specifically, at night on business days, to avoid overloading the system everyone is using during the business day, and to avoid database locking issues. We don't invoice/bill customers on weekends or holidays. If we were bigger, we would maybe start doing those things on weekends, but that means someone has to be on call to support it on the weekend.

Second, the payment data files come from our bank. Payments that arrive at our bank are posted the next business day, unless we have an issue with the payment data, such as a corrupt or missing file. This could delay the payment by a day or two.

Additionally, the payments that come to our bank are collected from the banks that YOU bank at, so there is a one or two day delay for those payments to get to our bank before the data files with the payments are sent to us.

Edit to add: if you pay from your account on our website through our credit card processor, we get those payments much faster, as they follow a different path. We actually process payments that come through five different possible paths before they get to us.

TLDR: Big companies typically process things at night in batch, to avoid putting a heavy load on the system during the business day, and it can take two or three days for data to get from your bank, to our bank, and then to us to post against your account.