Core Viewpoint - On October 21, a rare astronomical event will occur where Mercury and Mars will appear together in the southwestern sky after sunset, providing a unique opportunity for the public to observe this "celestial date" [1][15]. Summary by Sections Definition of "Conjunction" - In astronomy, "conjunction" refers to two celestial bodies having the same geocentric right ascension or geocentric ecliptic longitude. This is a visual phenomenon where the planets appear to come together from Earth's perspective, although they are actually far apart in their respective orbits [4]. Frequency of Conjunctions - Planetary conjunctions are relatively common, but the frequency varies among different planets. Mercury, being the closest planet to the Sun, has a maximum angular distance of only 28 degrees from the Sun. Mars has an average conjunction cycle of about 780 days, during which it approaches the Sun once, allowing for potential conjunctions with Mercury [7][8]. Observational Conditions - Approximately every 780 days, Mars will have a conjunction with Mercury. However, the specific number of conjunctions that can be observed is not fixed due to varying relative positions and speeds of the planets around the time of Mars' conjunction with the Sun [8]. The upcoming conjunction on October 21 is particularly favorable, with both planets having a significant angular distance from the Sun, around 22 degrees, making it a rare observational opportunity [13]. Visibility for the Public - For the public in China, after sunset on October 21, there will be a chance to see the two planets in the southwestern sky, approximately 5 degrees above the horizon. Mercury will have an apparent magnitude of -0.2, while Mars will have a magnitude of 1.5. There will be about a half-hour window for observation, with better conditions in southern regions compared to northern ones. While this celestial event can be seen with the naked eye, using binoculars will enhance the viewing experience [15]. Future Conjunctions - After October 21, there will be four more conjunctions of Mercury and Mars, with one particularly notable event in April 2026, where Saturn will also join, creating a triple conjunction that is highly anticipated [15].