Have been in southern areas (Oslo and south) in both spring and September and weather was perfect for me - mostly sunny, with temps mostly in the 60's or low 70's during the day - a little chillier at night. IMHO spring is better since days are much longer. If you go to the far north it will be colder. I had a light jacket and a couple of thin cotton sweaters - and really needed them only at night or out on the water.

But obviously averages over 100 years or so are going to be more accurate than my 4 experiences.

you will be safe in assuming high summer temps to be very rare - as is real cold.

we travelled to russia in late may and moscow was hot..tshirt weather...st petes the following week was raining and cold

in early june we were in sweden and it was cool instockholm
by the 2nd week of june we were in oslo heading out to geiranger and we were in layers...there was snow in parts of the tolls road and about 6 degrees
we were in oslo again mid to late june and it was never tshirt weather..it rained and was rather cool
probably around 17 degrees celsius

in finland helsinki in late may it was very cold for us
we were in ski jackets and hats
the wind in the city was horrid and it rained too

Hello Sallyjane,
I would say that it depends a lot on where in these countries you're talking about - from southern Denmark up to northern Norway/Finland it's about 2000 km so it varies a lot. Being from southern Sweden myself(Gothenburg), I would say that late May/early June is probably the nicest time of the year, normally, at least in mid/southern Scandinavia. Expect sun, often fairly warm days(around 20 C) and lots of flowers and nature at its best. Of course it can be cold and rainy, but normally I would say the weather is fine. The days are long, too, so you can really stay outside and see lots of things.
If you're going far north, prepare for possible snow and much colder.
Ragnar

I love cities, Ragnar, so I'll be in Helsinki (with a side trip to Tallinn), Stockholm, Oslo, and Copenhagen.

Thanks, everyone, for the info. Sounds like I picked a good time!

It is impossible to predict weather, at least here (in southern Finland). If I think about the last day of May, the day the school ends, I remember very different days. The day my son graduated was so hot (near +30°) that our Scottish guest said he had never felt so hot. The day my daughter graduated it was raining and around +15°. Nasty weather. Some years it is all summer, leaves full blown in trees and flowers everywhere. But I also remember one time it snowed when there was the"school is over" party. Take T-shirts but also a sweater or fleece and a wind proof jacket (not a thick one). Just in case.

And even if days may be warm the nights are still cold in May/June. Nights don´t usually warm up before Midsummer.

The other thing you need to consider is where you are from and what weather you are used to. My beau's neice came to visit us from FL and she was freezing the whole time she was in New York. Granted it was November - but temps were in the 50's - and she was cold even wearing a winter coat, hat etc - while I just had a leather jacket and was plenty warm. And our indoor temp is about 75 (old building and we have to be that warm for other apts to be the 68 minimum) and she kept asking us to make it warmer - even though she was wearing a sweater.