I will be in Florence Sat.-Mon. In September. Are the museums and/or shops closed any of those days?
Sunday and Monday some museums will be closed; the RS guidebook for Italy has a helpful list. Shops that sell to tourists will most likely be open.
Generally, many museums are closed on Monday especially if open over the weekend. However, you need to check with each museums as to their specific hours. If shops are closed it will be Sunday but in touristy areas they tend to open for the tourists. It just varies so it is hard to give a specific answer beyond a general response.