We are planning a trip to northern Croatia and Slovenia in the fall. Would it be better to go in mid-September or mid-October?
Mid-September is still in-season and the weather will generally be nice. Mid-October is off-season, so lodging prices outside larger cities tend to drop, crowds thin and seasonal businesses begin to close. Some days in October will be noticably cooler, and the resort towns in Istria will become very quiet. Larger cities like Ljubljana will be lively either month, and Bled and Plitivce could be stunning if in fall colors. There are trade-offs between those 2 months, so it really depends on what "better" means to you.
Thank you for your response. I'm mostly concerned with a lot of rain and businesses being closed. We do want to go to Istria, probably Pula and Rovinj. Do you know if most businesses be closed there by mid-October? Will we have a hard time finding a place to stay/eat?
I stayed in Rovinj first week of October a few years ago. Lodging was very inexpensive. I paid EUR 35 per night for a 2 room apartment with washing machine in the old town. Some places were closing down for the season, but several were still open. It was easy to get seated at restaurants. Not certain what might change by mid-October. Pula was very lively, whereas the hill towns I visited were very quiet. Weather was pleasant during the day but very cool and windy in the evening. Car is essential if you plan to visit more than Rovinj and Pula. I've also been in northern Croatia and Slovenia in mid-to-late September. Temperatures were warmer with occasional rainy days. Lake Bled was sunny and warm when I was there mid-September and cool at the end of September, although that could vary year-to-year.