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Binary supermassive black holes (SMBHs) are expected to be common from galaxy mergers. Their final inspirals and coalescences should produce gravitational waves detectable by pulsar-timing arrays and future space interferometers comma making them exciting targets for multi-messenger astronomy. Subsequent to the most recent proposal deadline comma the first candidate coalescing binary SMBH has just been discovered with rapidly decaying periods revealed by the remarkable optical and X-ray light curves (with the periods decreasing from virgulone year to virgulone month in the past three years). Multiple lines of evidence suggest that the system may be an uneven mass-ratio comma highly eccentric binary SMBH that will likely merge within virgulone to three years comma presenting an exciting opportunity for multi-wavelength and potentially multi-messenger observations to witness the first ever binary SMBH merger event comma justifying the mid-cycle request. New STIS UV spectroscopy may reveal evidence (or lack thereof) for a cutoff or notch in the NUV spectrum comma which may arise from gap opening in the circumbinary accretion disk comma and to measure the UV emission lines as tracers of accretion disk winds. Multi-band high-resolution WFC3 imaging is proposed to quantify the structural properties of the host galaxy comma enabling bulge stellar mass measurement through robust bulge-disk decomposition for better (total) SMBH mass estimation comma and to search for morphological evidence of a past merger. The program will independently test the binary hypothesis and provide the anchor for future follow-up campaigns. It has broad implications for multi-messenger astronomy and for understanding the cosmic formation and evolution of SMBHs.