The phrase "younger day" refers to a time in the past when someone was younger, perhaps recalling a particular period of childhood or adolescence. Some synonyms for this phrase could include "earlier years," "formative time," "childhood days," or "innocent days." These terms all evoke a sense of nostalgia and sentimentality about a time when life was simpler, and responsibilities were fewer. It is common to reflect on one's younger days when feeling nostalgic or when attempting to reconnect with a sense of childlike wonder. Whether reminiscing about summers spent playing outside or holidays spent with family, these synonyms capture the essence of the "younger day" sentiment.