Any of you experts know how often something like this occurs, visible to us?

I believe it is related to six year ecliptic latitude of Jupiter. These mutual phenomena of the Jovian satellites are more frequent as Jupiter crosses the ecliptic plane. See http://www.skyandtelescope.com/community/skyblog/observingblog/53840132.html "Right now the planet Jupiter is oriented such that its equator and the orbits of its four big moons are almost exactly edge-on to the Sun and Earth. This alignment happens every six years, on opposite sides of Jupiter's 12-year orbit around the Sun." - Kurt Table 1. Jupiter's Geocentric Ecliptic Latitude 1997 to 2027 Date Eclip Lat       Min Year 1997-Aug-27 0.15 x 1998-Aug-27 2.52 1999-Aug-27 3.77 2000-Aug-26 3.55 2001-Aug-26 2.43 2002-Aug-26 0.94 2003-Aug-26 -0.58 x 2004-Aug-25 -1.92 2005-Aug-25 -2.9 2006-Aug-25 -3.35 2007-Aug-25 -3.06 2008-Aug-24 -1.76 2009-Aug-24 0.5 x 2010-Aug-24 2.78 2011-Aug-24 3.8 2012-Aug-23 3.43 2013-Aug-23 2.25 2014-Aug-23 0.75 2015-Aug-23 -0.75 x 2016-Aug-22 -2.05 2017-Aug-22 -2.99 2018-Aug-22 -3.37 2019-Aug-22 -2.95 2020-Aug-21 -1.49 2021-Aug-21 0.87 x 2022-Aug-21 3.02 2023-Aug-21 3.81 2024-Aug-20 3.31 2025-Aug-20 2.08 2026-Aug-20 0.58 x 2027-Aug-20 1.0798   Table 2. Jupiter's Geocentric Ecliptic Latitude at 30 day intervals during 2009 Date Eclip Lat         2009-Jan-01 -1.13  2009-Jan-31 -0.83  2009-Mar-02 -0.50  2009-Apr-01 -0.16  2009-May-01 0.16  2009-May-31 0.41  2009-Jun-30 0.56  2009-Jul-30 0.58  2009-Aug-29 0.48  2009-Sep-28 0.36  2009-Oct-28 0.30  2009-Nov-27 0.35  2009-Dec-27 0.52  Ob-lon Ob-lat =     Apparent planetographic ("geodetic") longitude and latitude (IAU2000 model)of the center of the target disk seen by the observer at print-time. Light travel-time from target to observer is taken into account.  Latitude is the angle between the equatorial plane and the line perpendicular to the reference ellipsoid of the body (e.g., reflects body oblateness). Positive longitude is to the west.  For the gas giants Jupiter, Saturn, Uranus and Neptune, IAU2000 longitude is based on the "System III" prime meridian rotation angle of the magnetic field. By contrast, pole direction (thus latitude) is relative to the body dynamical equator.  There can be an offset between the magnetic pole and the dynamical pole of rotation.  Units: DEGREES