Hi Abood,
You have to be extremely careful with initialization of Inventory Management. Even if it's a weekend or even a public holiday, there are always few users working and also batch jobs might run. So usually posting cannot be prevented without really closing the system by locking the users and postponing the batch jobs.
I would really advice you to close the system and do the initialization of all (2LIS_03_BX, 2LIS_03_BF and 2LIS_03_UM) in one go.
Please have a look at the document Re-initialization of the Material Stocks/Movements cube (0IC_C03) with 2LIS_03_BF, 2LIS_03_BX and 2LIS_03_UM in BW 7.x for contains a step-by-step instruction and lots of valuable information.
Best regards,
Sander