ganzjährig
year-round
Lasting, available, or valid throughout the entire year.
Dieses Hotel hat einen ganzjährig geöffneten Außenpool.
This hotel has a year-round outdoor pool.

year-round
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year-round
Lasting, available, or valid throughout the entire year.
Dieses Hotel hat einen ganzjährig geöffneten Außenpool.
This hotel has a year-round outdoor pool.

The word is made up of 'ganz' (whole), 'Jahr' (year), and the adjective ending '-ig'. So it literally means 'relating to the whole year'.
Die Insel bietet ganzjährige Reisemöglichkeiten.
The island offers year-round travel opportunities.
'Ganzjährig' (year-round) can stand before a noun as an adjective or function as an adverb that refers to the verb. As an adverb, it remains unchanged and is not declined.
Das ist ein ganzjährig bewohnbares Ferienhaus.
That is a vacation home that can be lived in year-round.
This word is used very frequently in tourism, hospitality, and commerce. It often describes the availability of places, products, or services.
Der Campingplatz ist von April bis Oktober geöffnet, also nicht ganzjährig.
The campsite is open from April to October, so not year-round.
summer
'Sommer wie Winter' (summer and winter) and 'ganzjährig' (year-round) can both mean that something applies or happens throughout the whole year. 'Sommer wie Winter' is a fixed expression and sounds more figurative. It is often used when you want to emphasize that something is the same in every season. 'ganzjährig' is a normal adjective and sounds more factual. It works well for offers, opening hours, rules, or things that are always available. In terms of meaning, they are very similar here, but the style is different.

Das Schwimmbad bietet ganzjährig Kurse für Kinder an.
The swimming pool offers courses for children year-round.
Der kleine Laden ist Sommer wie Winter geöffnet.
The small shop is open summer and winter.