I will attend a mid summers eve celebration in Sweden. How should I
think about and how should I behave to "melt in" ?

Scandinavia
In the far north the longest day of the year (twenty to twenty-four
hours of sunlight as compared to zero to six mid-winter) is a joyous
occasion! In Finland the day is called Juhannus Day and is
conveniently fixed on a Saturday so that everyone can spend it in the
country. On Midsummer's Eve the celebration begins with a sauna,
followed by the midnight lighting of great bonfires on lake and sea
shores and outdoor dancing through the night. The birch tree takes the
place of the Christmas evergreen; great branches are cut and used to
decorate cars, homes, and churches. Superstitions abound, such as you
must have a birch tree in the house to insure future happiness, and
you should hang a birch wreath to your cow's horns for protection.
Hundreds of fires can be seen on the highest promontories throughout
Finland and Norway with shadow-like figures dancing around them. Boats
decorated with flowers and green branches float down fjords, giving
passengers a good view of the lighted hilltops.

In Sweden, where the festival is called Midsommar, houses are
decorated inside and out with posies, wreaths and flower garlands. A
midsummer pole, similar to the maypole is erected in a field and
decorated with leafy branches and flowers. People of all ages gather
around the pole to dance, led by fiddlers playing classic old tunes
and children's favorites. In Sweden, as in many other countries, the
magic of Midsummer includes divining the future, especially one's
future spouse. The young girl who goes out into the fields, and (in
silence) picks seven different kinds of flowers and places them under
her pillow that night, will dream who her husband will be.

Ah, the song. It's not Santa Lucia, but yes, it's the fesival of holy
light (i.e., Santa Lucia).

Well, in Sweden that was a worthy old tradition, but even back in the
late fifties in Leksand the girls were doing more forward things to
see who could be their mate (maybe not for life).  But that might have
gone out a style  - or not be limited to Midsummer night.
Be prepared for beer and aquavit.  If all the Swedes are having a good
time, they are not going to notice if you blend in, unless you make an
effort not to.
1/2 Swede Myoarin

Melting in will be easy. Midsummer eve is the highlight of the year
for many Swedes, so everyone will probably be happy and in celebration
mode.

We usually eat herring and potatoes for lunch, and barbecue in the
evening. And be prepared to drink lots of beer and "snaps" throughout
the entire day.

There are usually no dress codes on midsummer eve celebrations. Most
people wear casual clothes in light colors. Have a nice evening!

Greetings from Andreas in Stockholm, Sweden.

Great link to answer your question
www.religioustolerance.org/summer_solstice.htm