We are staying in Tuscany and plan on taking a day trip to Florence. No museum tours, just wandering around. Would a Friday or a Saturday be better for this?
Either day should be good. Where in "Tuscany" are you staying? Florence is the capital of Tuscany so transportation shouldn't be a problem.
If traveling by bus, service is usually similar on Friday and Saturday (confirm locally) but nonexistent on Sundays (except for the most popular routes, such as Siena-Florence). Nothing is closed in Florence on either day. Streets and restaurants likely to be crowded on both days.
In the summer months (end of June to beginning of September) many shops are closed on Saturday afternoon as so many Florentines spend their weekends at the seaside and business gets slow. If visiting in summer and shopping is essential, Friday is better than Saturday.