Early October is late summer on the Mediterranean. Temperatures are usually in the 70's-80's degrees Farenheit, and it is often sunny. There is always a chance of breezes or rain.
See also the travel section: the town of Sitges is about 45 minutes by car and 1 hour 15 minutes be train from Barcelona. If you are not staying at the wedding hotel, it is possible to walk or take a taxi to and from the event within Sitges.
YES. RSVP is not yet available, but you will receive the formal invitation with a deadline for RSVP in the months to come. This deadline will be final, and any guests who have not RSVP'd within the specified time period will not be able to attend the event. Please be mindful of the challenge of planning this important event, and make sure to RSVP "yes" if you plan to attend!
If you have received a save-the-date, you will receive an invitation with RSVP instructions and the RVSP deadline at a later date. You can also check back on the wedding website for information on how to RSVP online.
https://travel.state.gov/content/travel/en/passports.html New passport application processing times are supposed to be 6-9 weeks, though may take longer (up to 13 weeks by some reports). Renewals may take less time than that. Make sure that your passport is current and good for at lease 6 months beyond your trip to Barcelona! As a US citizen, you do NOT need to apply for a visa to travel to Spain; however, you do need to register for the ESTA program, which can be done online. See below for a link...
Most likely yes.. Check with your carrier about pricing and offers. Most modern Smartphones are able to operate on European phone networks, for a fee ;)