The South Caucasus is three countries stacked against a mountain range, and the mountains decide the calendar. A month either side of your plan can mean a different trip entirely.
Guidebooks tend to say spring or autumn and leave it there. That is not wrong, but it hides the part that matters: at these latitudes the limiting factor is altitude, not the month. A valley town can be warm while a pass two hours away is shut.
The lowlands are at their best — green, mild, and empty of other travellers. The mountains are not. High passes stay closed well into June in a heavy year, and the roads that cross them are the ones that make the region worth driving. Snowmelt also means brown rivers and the occasional washed-out section on secondary roads.
What spring does give you is the wildflower window in the alpine meadows, which lasts a few weeks and is genuinely worth planning around.
Hot in the valleys, properly hot on the Azerbaijani plain, and pleasant only once you climb. This is the season to be in the mountains and nowhere else. Everything is open — passes, high villages, the remote church routes — and accommodation in the highlands fills up, particularly at weekends when people are escaping the cities for the same reason you are.
The Black Sea coast is the exception: humid rather than hot, and busy with domestic holidaymakers through July.
The best compromise, and the reason most experienced travellers come at this time. Passes are still open, the heat has broken, and the grape harvest runs through September and into October depending on region and altitude.
In wine-growing valleys the whole village is doing it. Turn up willing to help and you will be fed better than in any restaurant.
Low sun, clear air after the summer haze, and the first snow on the peaks while the valleys are still warm.
Passes close, mountain villages become difficult or impossible, and some of them empty out entirely until spring. What opens instead is the ski season, which is cheap by European standards and improving quickly, and the cities, which are at their most ordinary and therefore their most interesting — nobody is performing for visitors in February.
Daylight is short and weather moves fast. Plans need slack in them.
July to September Avoid November to May — the passes decide this one, not you.
Mid-September to October Spring gives you vineyards with nothing happening in them.
November to March August is the one month to avoid for this.
Late May to June A few weeks only, and gone by July.
First, a pass being technically open and a pass being sensible are different things, and local advice on the morning is worth more than any website. Second, border crossings in the region have their own politics and their own opening hours; the route that looks obvious on a map may not be a route at all. Check before committing to it.
For the roads themselves see routes, and if you are coming in harvest season, what to eat changes completely with the month.