Launch windows for Venus transit; how to convert heliocentric longitudes to date? Being a bit familiar with celestial mechanics, I know that the Hohmann transfer orbit is the quickest way to transfer between two circular orbits of different radii around a central body in the same plane. JPL's Let's Go to Mars! Calculating Launch Windows explains the geometry and offers a very intuitive guide how to calculate the travel time to Mars, and this can easily be repeated for Venus.
Just for fun, I would like to mark all launch windows for a travel to Venus on a calendar, similar to the list compiled by Don P. Mitchell
1991, Jun 5     Unused Type I window
1993, Jan 5     Unused Type I window
1994, Aug 9     Unused Type I window
1996, Mar 20    Unused Type I window
1997, Oct 15    Cassini flyby   

Question: I have a probably minor (astronomical) issue: I am struggling how to convert heliocentric longitudes into real dates? I guess I am missing the English terms here. I would appreciate a small reminder how to proceed.
Heliocentric longitude is a direction, not a date.
A Heliocentric longitude of 0 is the direction from the sun towards the vernal equinox (currently in Pices)
You can ask for the date on which the Earth is at a particular Heliocentric longitude.  A careful calculation would require accounting for the eccentricity of the Earth's orbit, and also adjusting for the oddities of the calendar, but since when the Sun in on the vernal equinox (About March 21), the Earth will be at 180 degrees of heliocentric longitude, and longitude will increase by 360/365.24 degrees per day.