Lightshare on OpenSim is the equivalent of Second Life's windlight functionality, which is fully supported on OpenSim.
Generally, this is done with your viewer to change your personal viewing experience or to set a region -if you are the owner- for all visitors to enjoy.  Here are the instructions for the Phoenix viewer Find the section "LightShare Scripting" and follow the links.

Also check out this link:

Changing the windlight settings directly in your Firestorm Viewer

Any landowner can easily set up Phoenix Windlight settings with a simple change to the description of your land in the About-Land tab.

Just add a line like this at the end of the parcel description in the About Land -> General tab:

 /*Sky:"[preset]"Water:"[preset]" RegionOverride*/

Example: If the sky preset is to be 'Incongruent Truths' ( my favorite) and the water preset 'Pond' and the parcels description is ”My place” then the parcel description would be like this:

Your land description

/*Windlight Sky:"Coastal Afternoon" Water: "Naco's Lake Water" Sky @ 600m to 5000m:"A-6PM"*RegionOverride*/
Created by Oni Kenkon Creations
Nebadon Izumi "UCI edu1"

5 seconds after someone using Phoenix enters a parcel, the viewer will prompt for permission to change their settings, or to ignore them. For you, the settings will auto-apply if the land is owned by you, the land is owned by a friend, the land is deeded to a group you are a member of, or you previously granted permissions to change to the same land owner on this or another parcel.

You can also set parameters for altitude, such as for a skybox, independently of the land settings at ground level..

For more information on various options, check the Phoenix viewer blog at this link.