Hi! I search on the French Web and that's what I found c:

In the past language of French, we can say: en l'été , en l'hiver, en l'automne, but en le printemps

Today we say (by the contraction of en + l' ): en été, en hiver, en automne, they just loose the l' because we can't say au hiver, au automne and au été.

(and by the contraction of en + le ): au printemps

So, the answer of the question is historical! (I learn English. I can make some errors, hope you will understand! )

You say en hiver, en automne, en été...they all begin with vowels. But in the spring is au printemps.

"Spécialement" (especially), "particulièrement" (particularly) & "surtout" (above all) are more or less the same. But "notamment" (notably) & "principalement" (mainly) have a slightly different meaning.