Scandinavia in Spring? Crazy?

Scandinavia in May have heat, you hope. I went to southern Norway in late July and it was warm during the day and cool in the evenings. We went north and I layered in late July when we went on Norway in a Nutshell. It may get warm during the summer but not like Italy or Spain or even France. I would not go in late April, too cold, too rainy. I would start looking at the weather for previous years to see what kind of weather there is for the countries you plan to visit. Norway, Sweden, Denmark and even in the summer or May bring a rain jacket, and layer and jeans.

Have a great time.

I'd check whether key things you want to see are open or closed in April. Closures may be few but, for instance, looking at Rick's guidebook listings for open-air folk museums (which I love), those closest to Copenhagen and Helsinki don't open until May, while others just have shorter hours off-season (e.g. close at dusk and/or close an extra day per week).

Trains tend not to be seasonal but buses and boats more so. If demand is growing, service may slowly increase. For instance, Rick's book mentions Norled's Bergen-Sognefjord Express boat running 1x/day in April vs 2x/day in May, based on research in 2018; however in 2019 it was scheduled 2x/day from April 1 to Sept. 30 (and 2020 schedules are not yet online).

Not sure what Easter events might be happening, but in early April, you’d be there for Easter. And, depending how far north you’d go, Northern Lights could still be viewable. Any chance you’d be up for skiing, cross country OR downhill?

Depends on what you want to do and where. You shouldn't have seasonal limitations in urban and developed areas with the exception of national holidays.

You can check opening hours/days for must see attractions. As a for example, Oslo's outdoor Norwegian Folk Museum's winter hours through April are 11:00 to 16:00 expanding to 10:00 to 17:00 May 1: https://norskfolkemuseum.no/en/hours-fees-directions
Similarly operating hours for the Bygdøy Ferry are longer starting May 1.
Early May in Oslo would be a bit better than late April

However, if you are looking fo mountain experiences, staffed mountain lodges and huts shut down shortly after Easter (end of ski season) then reopen for the summer, ie. July (beginning of hiking season).

BTW it seems as if Norwegians abandon the cities and head for the mountains Easter week than take the Monday after Easter to return. Trains are packed the weekend before Easter and the cities are empty from Easter Thursday to Easter Sunday.

Would we be out of our minds to visit there in late April? it is too cold, to o dark??

It won’t be too dark. Keep in mind that from about March 20 to September 20, Scandinavia has more hours of daylight than we have in the continental US.

Late April will be fine in the cities. If you happen to be in Sweden on April 30, it is Walpurgis Night (Valborgsmässoafton) and there will be bonfires that evening.

Hi, we have typically traveled to Europe in late March early April
(mainly to avoid summer crowds) ,however RS guide says the best time
for Scandinavia is May and after..... Would we be out of our minds to
visit there in late April?

No, not at all crazy. Sound to me like the RS guide is misleading you.

It is too cold, to o dark??

Not dark at all (apart from nighttime), april is closer to the summer solstice than the winter solstice. Cold? Depends on how you define too cold. And where you plan to go. Remember that Scandinavia is large and from south to north it is almost 2000 km. So in late april there can be 20-25°C in the south while you can go skiing in the north. But the nights can be chilly even in the south, maybe down to 10-15°C. The amount of rain varies a lot from area to area, but in general the Norwegian coastline sees the most rain. The northern lights might be visible, but late April is a bit late for seeing them.

The good thing about visiting in April is that it is very much low season in the tourist areas so you will not face any lines anywhere. The bad thing is that it is low season and opening hours will be shorter than in the summer. But then again, if you come during Easter things might be different.

For mountain experiences, there are some mountain station that are open all year, but many will be closed in april. Valborgsmässoafton is indeed a good day to see some Swedish traditions even if it's usually a bit lowkey with bonfires and maybe a choir. The exception are university cities and especially Uppsala where the celebrations themself can be an attraction.