  We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio
source located at the position of the supermassive black hole Sagittarius A*
(Sgr A*), collected during the 2017 April 5--11 campaign. The observations were
carried out with eight facilities at six locations across the globe. Novel
calibration methods are employed to account for Sgr A*'s flux variability. The
majority of the 1.3 mm emission arises from horizon scales, where intrinsic
structural source variability is detected on timescales of minutes to hours.
The effects of interstellar scattering on the image and its variability are
found to be subdominant to intrinsic source structure. The calibrated
visibility amplitudes, particularly the locations of the visibility minima, are
broadly consistent with a blurred ring with a diameter of $\sim$50 $\mu$as, as
determined in later works in this series. Contemporaneous multi-wavelength
monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near infrared
and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by
Chandra, one at low significance jointly with Swift on 2017 April 7 and the
other at higher significance jointly with NuSTAR on 2017 April 11. The brighter
April 11 flare is not observed simultaneously by the EHT but is followed by a
significant increase in millimeter flux variability immediately after the X-ray
outburst, indicating a likely connection in the emission physics near the event
horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its
historical spectral energy distribution and find both the quiescent and flare
emission are consistent with its long-term behaviour.
