I don't know if there is a best day and time to go. The last time I was there, it was a big crowd and somewhat of a long line. However, the workers there help move things along at a good pace. I took my son to get his learners permit, and I think the entire process took an hour or so. Just be prepared to pack your patience if you go.
It's really a gamble. I'd say don't go on a Tuesday because they are closed Mondays. I'd say Wednesdays through Fridays the earlier the better.
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