From what your saying here I can pickup that you are using the in game editor. Applying your own modifications etc and then hitting save?
You will notice that when you go to you missions folder < C:\Documents and Settings\XXXXXXX\My Documents\ArmA 2\missions
That you have a folder relating to your mission, in my case . co40_Carrier_Domination_1_17TA2_R.Chernarus
When you save in the map editor (within the game) it will not save back to this file always. You will probably find that it has created a new folder, and within it only has 1 file 'mission.sqm'
All you need to do is copy that particular file, and paste it into your original mission directory. You will be asked if you want to overwrite the current file, click yes. This will fix your problem.
The reason it is always dark is without a description file the map does not have any variables such as time of day, number of missions etc, so it used the default mission time of 00:00am.
Hope this helps.
Z