Zweimal im Jahr ließen wir indessen das rote Futter durchleuchten – einmal im Frühling und einmal im Herbst.
Im Herbste zechten wir als Weise und taten den köstlichen Weinen, die an den Südhängen der Großen Marina gedeihten, Ehre an.
But twice a year we gave a glimpse of the brilliance of its red inner lining - once in the spring and once in the autumn.
In the autumn we feasted like sages and did honour to the exquisite wines in which the southern slopes of the Marina abound.
Dative forms with the ending -e, known in German as the Dativ-e (dem Gotte, dem Manne) are mostly restricted to formal usage, but widely limited to poetic style. Such forms are not commonly found in modern texts, except in fixed expressions (such as im Stande sein: to be able) and for certain words (e.g. (dem) Hause, Wege or Tode) which are, however, quite numerous; in these cases, omitting the -e would be similarly unusual.
I checked various editions like this one: I always found the same text.
My best guess: Jünger wanted to avoid the repetition of the words im Herbst. But I suspect some rhythmical reasons.