Any suggestions for seeing an Opera in Rome or Venice? Can I buy tickets a day or so ahead when I'm there, or do I need to purchase online well ahead of time?
Not sure when you're going but you'd need to see if there actually any Opera Performances scheduled for your dates. Whenever we travel to Europe I check a website for any and all concerts & performances at: www.classictic.com We've been very very pleased with their services for events in Barcelona, London and Paris. Good luck!
Opera seasons vary from city to city, so do be sure to check. Another option in case you can't find any opera is to look for concerts. And don't rule out the smaller opera companies. There's one in Venice that put on a wonderful shoestring production of La Boheme at Scuola di San Giovanni Evangelista. They were chorus members from La Fenice, the big Venetian opera house.