Understanding Mercury's Orbital Period
Mercury completes one full orbit around the Sun in about 87.97 Earth days. That is the sidereal orbital period, which is what most people mean when they ask about its revolution. The synodic period, or the time between successive oppositions as seen from Earth, is about 115.88 days. These two numbers are different because Earth is also moving around the Sun while Mercury does its lap. When I first worked with planetary orbital data, I kept confusing sidereal and synodic periods for Mercury. The problem was compounded by Mercury's high orbital eccentricity of about 0.2056. That means its distance from the Sun varies significantly between perihelion and aphelion. Kepler's second law kicks in hard here. Mercury moves much faster at perihelion than at aphelion, so its instantaneous orbital speed ranges from roughly 59 km/s near perihelion to about 39 km/s near aphelion. The average orbital speed is around 47.87 km/s. If you are doing anything that requires precise timing, like planning an observation window or calculating a spacecraft flyby, using a single constant speed for Mercury will give you errors. I ran into this when modeling encounter trajectories. The naive approach using average velocity put the probe off by several hundred kilometers at the point of closest approach. Switching to a two-body solution with proper perturbation terms fixed it. Even then, I had to add Mercury's own gravitational parameter adjustments, which are tiny but matter for high-precision work.
The exact value of 87.969 days comes from decades of radar ranging and spacecraft tracking. MESSENGER and BepiColombo both provided refined measurements. Before radar, estimates varied widely because Mercury is always close to the Sun in the sky, making optical observations difficult. There is another nuance worth noting. Mercury's rotation is locked in a 3:2 spin-orbit resonance. It rotates exactly three times on its axis for every two orbits around the Sun. That gives a solar day on Mercury's surface of about 176 Earth days, which is twice its orbital period. This resonance is stable because of Mercury's eccentric orbit and the tidal forces from the Sun. It is not a coincidence, and it matters if you are thinking about surface conditions or thermal cycling. Orbital period values you see online will sometimes differ slightly depending on the ephemeris source. JPL Development Ephemeris DE440 gives one set of numbers. INPOP19a gives another. The differences are usually in the range of a few seconds over the course of an orbit, but if you are doing something like timing a transit or planning a mission arrival window, that precision matters. Always cite which ephemeris you are using.
For most general purposes, saying 88 days is fine. If you need more precision, use 87.969 days sidereal. Just be clear about which period you are referencing and what your tolerance for error is.
