Wikipedia notes evidence that the summer solstice has been culturally important since the Neolithic era.
The Neolithic, or New Stone Age, saw major changes in human life, including farming, animal domestication, more permanent settlements, and the construction of large communal monuments. These developments made seasonal cycles especially important: communities needed to track planting, harvesting, migration, and religious festivals tied to the changing year.
Archaeological sites from and around the Neolithic period include structures whose axes or openings may correspond to solar events, including the summer solstice. Such evidence does not mean that every monument functioned as a precise calendar or observatory. Instead, repeated attention to solar positions can reveal that the sky formed part of a society’s practical and symbolic world.
The answer is sometimes confused with the Bronze Age because several famous monumental landscapes were expanded, reused, or given new meanings during later periods. Paleolithic peoples also observed natural cycles, but the specific historical claim in the question identifies the Neolithic as the era for which evidence of summer-solstice cultural importance is noted.