der Sommer
summer
The warmest of the four seasons, which comes between spring and autumn.
In diesem Sommer wollen wir eine lange Fahrradtour machen.
This summer, we want to go on a long bike tour.

summer
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summer
The warmest of the four seasons, which comes between spring and autumn.
In diesem Sommer wollen wir eine lange Fahrradtour machen.
This summer, we want to go on a long bike tour.

If you want to express that something happens during this season, use the preposition 'in' with the dative: 'im Sommer' (in summer).
Im Sommer gehen wir oft im See schwimmen.
In summer, we often go swimming in the lake.
The adjective 'sommerlich' (summery) is derived from the noun 'der Sommer' (summer). It describes things that are typical of summer, such as the weather or clothing.
Heute herrscht ein schönes, sommerliches Wetter.
Today we have lovely, summery weather.
The phrase 'Sommer wie Winter' (summer and winter) is a fixed expression. It means 'all year round' or 'no matter what season it is'.
Das Hotel ist Sommer wie Winter geöffnet.
The hotel is open summer and winter.
This meaning is metaphorical and belongs to a poetic or elevated style. It is used to describe the prime or high point in a person's life.
Im Sommer seines Schaffens schuf der Komponist seine berühmtesten Werke.
In the summer of his creative life, the composer created his most famous works.
winter
'der Sommer' and 'der Winter' are two opposite seasons. Summer is the warmest time of the year and comes between spring and autumn. Winter is the coldest time and comes after autumn and before spring. The contrast is especially important for weather, clothing, travel, and leisure activities: in summer, people often think of warmth and long days, while in winter they tend to think of cold, snow, or short days.

Im Sommer fahren viele Familien ans Meer.
In summer, many families go to the seaside.
Im Winter trägt sie morgens einen dicken Mantel.
In winter, she wears a thick coat in the morning.