Thursday carries Jupiter's significations: fortune, growth, expansion, generosity and the pursuit of meaning through law, philosophy or higher learning, following the qualities classical astrology assigns to the greatest of the seven visible planets and, historically, the greater of its two benefics alongside Venus. Older astrological and magical handbooks recommend Thursday for undertakings concerned with wealth, legal matters, education, and any enterprise meant to grow beyond its starting scale.
Because Jupiter was read as an almost uniformly favourable influence, Thursday accumulated a reputation in folk tradition as one of the luckiest days of the week for beginning new ventures, second in most accounts only to Sunday's own association with fresh starts, and the pairing of a Thursday working with a Thursday hour of Jupiter was considered a particularly strong combination for anything meant to expand or prosper over time.
Sources A History of Western AstrologyNicholas Campion with Grimoires: A History of Magic BooksOwen Davies