I'll be on the Istanbul in 7 days tour from May 25-31 and I'm wondering what type of clothing I should plan on bringing?
It will be getting warmer so summer weight clothing with a sweater or jacket for the evenings. Comfortable shoes or sandals that you can walk in - there will be a lot of walking. Istanbul is a cosmopolitan city and women wear either fashionable western clothes or more traditional Muslim dress. Short shorts and tank tops are not a good idea! Mosques require slightly more conservative clothing and its a good idea to carry a scarf or a pashmina to cover bare shoulders or head. The Blue Mosque did not require covered hair but it has been some years since I was there. Your guide should give you recommendations as to appropriate wear for whatever you are doing that day. Istanbul is a wonderful city and you will enjoy it.
I think that if you wear in Istanbul in May what you would wear at home in Boston in May, you'll be fine.
Be comfortable and conservative,