Final answer:
The arrival of summer in medieval times was celebrated around the summer solstice, occurring around June 21st, known as midsummer day. Celebrations were linked to the agricultural calendar, and observing the solstice involved monitoring the Sun's position in relation to fixed landmarks.
Step-by-step explanation:
In medieval times, the day that celebrated the arrival of summer was typically around the summer solstice, which occurs on or about June 21st. This day, also known as midsummer day, was marked by various cultures as a time to celebrate the longest days of the year and thank the gods for the warmth and light provided during the summer months. Such celebrations were linked to the agricultural calendar and were critical for medieval societies to track the changing seasons for planting and harvesting crops.
The observance of the solstice entailed tracking the Sun's position as it moved northwards in the sky, reaching its maximum at the summer solstice. This was observed by noting where the Sun rose and set in relation to fixed landmarks over several weeks. Knowledge of celestial movements was also reflected in calendar reforms such as the Gregorian Calendar, which improved upon the Julian calendar to more accurately align with the solar year and maintain the timing of important Christian festivities like Easter.