auspicious date
A classical four-character compound meaning a lucky or celebratory day; used in formal writing, poetry, or ceremonial contexts.
Hold a banquet on an auspicious date.
吉日 is the common modern word for 'lucky day'; 嘉辰令月 is a rare, classical yojijukugo with a more elevated, literary tone.
A yojijukugo of classical Chinese origin, combining 嘉 (fine, auspicious), 辰 (time, day), 令 (good, excellent), and 月 (month). The exact historical derivation is uncertain, but it is used in traditional Japanese writing to refer to a propitious date.