When does shoulder season begin?

I haven't spent enough time in that area in the fall to give you a reliable opinion, but I have two thoughts:

• You could use a hotel website like booking.com to see what rates look like for different one-week periods. Higher cost would imply that the hotel expects to busier. Rates can definitely vary between weekdays and weekends; that's why I suggest looking at the cost of one-week stays.

• Sometime between late September and mid(?)-October, autumn will begin moving in, meaning cooler temperatures and greater likelihood of rain. When that happens can obviously vary from year to year as well as from place to place. It is a given that the days will be getting shorter every week in all locations. If you prioritize avoidance of heat as well as crowds, the latter part of your time-window may be a reasonable choice. I prioritize longer days and avoidance of chilly, wet weather, so I would prefer to end my trip by early October at the latest. It helps that I spend a lot of time in less-crowded places, so dealing with the (probable) larger crowds in September isn't a deal-breaker for me.

Based on my single recent experience in the Balkans (2015), October was not ideal. It was getting rainier. The temperature was still pleasant except for the day I got totally soaked (October 11, Zagreb). That was chilly and not fun.

Someone just posted that they could not believe how crowded Rome was Sept 22.
https://community.ricksteves.com/travel-forum/trip-reports/returning-from-italy-fewer-warm-fuzzies

Shoulder season can be a bit misleading. Once September gets going, there are many special events, trade shows, conventions that fill hotel space and boost prices. I see hotel prices lower in Venice in August than in September.

Local schools in Central start their summer holidays from the middle of June, whereas Northern European schools start mid July. Shoulder season is therefore early June. September is also shoulder season in Croatia. The weather there usually breaks towards the end of September. October can be nice or can be quite wet and considerably cooler.

The season seems to be longer in Italy, as places stay open longer.

I was in Croatia the third week in September and it had a dead, out of season feel.

I think the lower number of visitors during what used to be called "shoulder season" (April-May, Sept-Oct) may be less obvious and meaningful now. Those periods are probably still less crowded than summer in most European locations, but the most popular spots are often very busy, even mobbed, during those months. The question is: Is less-but-still-dreadful crowding worth the weather risk (in some places) and the shorter days (everywhere in the fall)? For me, the answer is "No", but for plenty of other people, the answer is "Yes". I think the trade-offs are becoming less attractive, though.

My first visit to Croatia was in early October. The weather varies year to year of course - but while I was there, the weather turned from warm and sunny to cold and rainy. It was warm (and busy) in Dubrovnik while I was there, but the rain came by the time I got to my next stop Korcula, which in early October seemed dead and about to close up for the season (restaurants were open at night but you had to walk up to them to see, it didn't look like they were). Granted this was 2009, and I think tourism has increased a bit since then. But I usually don't enjoy visiting a place where people aren't out and about and I am one of the only people eating in a restaurant.

But if you are interested in excursions, note that the beginning of October in Croatia seems to mean reduced options - shorter hours, fewer ferries, etc. On the upside, maybe fewer tourists. And although the weather could be perfect (or it could be rainy in September), the odds of poorer weather increase as you get into October.

If I were going to Croatia again in the fall, I'd aim for the last two weeks of September myself.

Shoulder season isn't the same everywhere.

In Croatia, May and September are probably the only shoulder season months, possibly some of June. June, July and August are high season. Outside of those months travel seems to drop off nearly completely.

In Southern Spain, July and August aren't high season because it's too hot. April and May, September and October are part of high season because it's the best time to be there.

Rome is never low season, there is a constant crowd of visitors. I'd rather be there in November than July or August.

September is a beautiful month in Germany but it starts getting cold sometime in October and stays cold until spring.

We were in Dubrovnik Nov 3-6 in 2014. It was pleasant, although cool. We needed light jackets. The crowds were nonexistant. Restaurants were open. We also went to Sarajevo on the same trip, although more like Oct 23-26. Again, inexpensive hotels, no need for reservations in restaurants, etc. It was a great trip.

We recently visited Croatia September 8-22. The weather was delightful—warm enough to swim in the ocean, sunny, and warm but not hot air temperatures. However, it can still be crowded during this time. I reviewed cruise ship schedules and planned our trip so that we would be in Split and Dubrovnik when there were no ships in port. It made a huge difference. We met some young women in Vis who said Split was unpleasant because of the crowds from cruise ships—a totally different experience than we had.

Thank you this was helpful! We are trying to decide if we should go last week of August (the pro is that we will be with family who can help with kiddos, cons are hot and crowded) or mid September (no help with kiddos but more pleasant). Leaning toward mid September.