EDIT: 24/05/21 - I am using CSP 1.74.33 preview so if you have issues when it repeats itself with daytime being dark just toggle the repeat icon on/off and that fixes it - or turn off all together.
Requires:
Content Manager (Full version) free lite version here: https://assettocorsa.club/content-manager.html
SOL 2.1: https://www.racedepartment.com/downloads/sol.24914/
Custom Shaders patch v1.69: or higher
Full (External track) RainFX the X4FAB Patreon version is required
Installation Instructions
Download/UNZIP/COPYTO; YourPC:\SteamLibrary\steamapps\common\assettocorsa\apps\python\sol_planner\plans
Set time multiplier to x60 and laps to 6 which will give you a 24-26 min race 1600hrs to 1600hrs using either circuit which can be found here;
Le Mans 2017: https://www.racedepartment.com/downloads/circuit-24h-lemans-2017.2482
Circuit de la Sarthe - Le Mans (leBluem): Google Search 'sx_lemans_v1.8.7z'
Further Help:
