This is the 8481st most frequent Dutch word.
hartje
"Hartje" means "little heart" or "the center (of something)."
Here, 'hartje' is used to mean the central or most important part of a place, idiomatically referring to the city center.
Het hartje van de stad is altijd levendig.
The heart of the city is always lively.
In this context, 'hartje' denotes a small or cute heart, often used in drawings or decorations as an affectionate symbol.
Ze tekent een rood hartje op de kaart.
She draws a red little heart on the card.
In this sentence, 'hartje' is used figuratively to represent the core or height of a period, in this case, summer.