το θέρος means το καλοκαίρι (the summer) in ancient Greek.
In modern Greek the word θέρος is not used as it is.
We use only the adjective θερινός, -ή, -ό (which comes from the noun "θέρος") and it is a formal word instead of καλοκαιρινός, -ή, -ό.
Example:
Πότε αρχίζουν τα θερινά μαθήματα στο πανεπιστήμιο της Αθήνας; = When do the summer courses start in the university of Athens?
Ανυπομονώ για τις καλοκαιρινές διακοπές! = I am looking forward to the summer holidays!
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.
