Final answer:
The question likely pertains to the feast day of a saint named Gregory, which was not directly answered in the provided information. The details surrounding the history of the Gregorian Calendar and its introduction by Pope Gregory XIII in 1582 were discussed, highlighting the importance of these reforms in relation to religious celebrations and the precise timing of events such as Easter.
Step-by-step explanation:
The question regarding Gregory's feast day seems to be related to historical dates and their significance within religious or cultural contexts. The specificity of 'Gregory' may also imply a link to saints and their feast days within the Christian Church. The Gregorian Calendar, instituted by Pope Gregory XIII in 1582, was a significant reform of the previous Julian calendar to closely match the solar year. Despite the details provided, the information does not directly mention the feast day of Gregory. It would require additional context to identify which 'Gregory' is being referred to, as there are multiple saints with that name celebrated on different dates.
However, if we consider Pope Gregory XIII in relation to the Gregorian Calendar, it is noteworthy to mention the adoption and adherence to this calendar system which corrected the date of the vernal equinox back to March 21 and changed the rules for calculating leap years, ensuring that the average length of a year more closely approximated the tropical year. The impactful changes including the dropping of ten days from the calendar to adjust the equinox and the new rule for leap years that centuries must be divisible by 400 to be considered leap years have standardized the determination of dates like Easter and other cyclical celebrations in the Christian Church.
Regarding the celebration of saints' days, the practice of commemorating saints on specific days is common within many Christian denominations. These celebrations can include religious services, feasts, and other traditions. Unfortunately, without further context, we cannot definitively determine the precise date of Gregory's feast day, but it does indicate the importance of calendar reforms in the alignment of religious observances and festivities with solar events and seasons.