Signs, signs, everywhere signs. I would set signs out for the next 30 days letting folks know that the lobby will be closed on Saturdays beginning 30 days from now. If there are any services that will now not be performed by drive through tellers, be sure the staff is ready to tell people about them. If those services will be provided at another location, be sure to let your staff know where to instruct folks to go on Saturdays.
The last thing you want is a surprised customer who does the same transaction every Saturday or even one Saturday a month come in without prior notice that they can't do that transaction on Saturdays.
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Only two things that money can't buy, that's true love & homegrown tomatoes