we are visiting Istanbul in April and have just found that there is a public holiday on Monday,April 23.
Is everything shut down that day - including the Grand Bazzar or the ferries ??
Do you have RS Istanbul Guide Book yet? If so, good. Check in there and see if he mentions it. If not, you should get one as it has lots of good information that we used when we spent time there 2010. I would go back to Istanbul in a minute. Great place, people, and had a great time.