Code:
$ cat file
Mg_Message_count,1-Aug-12,46
Mg_Message_count,2-Aug-12,48
Mg_Message_count,3-Aug-12,48
Mg_Message_count,4-Aug-12,48
Mg_Message_count,5-Aug-12,48
Mg_Message_count,6-Aug-12,48
Mg_Message_count,7-Aug-12,42
Mg_Message_count,20-Aug-12,24
Mg_Message_count,21-Aug-12,24
Mg_Message_count,22-Aug-12,24
Mg_Message_count,23-Aug-12,24
Mg_Message_count,24-Aug-12,24
Mg_Message_count,25-Aug-12,24
$ awk -F "[-,]" '{if(a){if(($2-a) > 1){p=$1",";q="-"$3"-"$4",0";a++;for(i=a;i<$2;i++){a++;print p""i""q;}{print}}else{a=$2;print $0}}else{a=$2;print $0}}' file
Mg_Message_count,1-Aug-12,46
Mg_Message_count,2-Aug-12,48
Mg_Message_count,3-Aug-12,48
Mg_Message_count,4-Aug-12,48
Mg_Message_count,5-Aug-12,48
Mg_Message_count,6-Aug-12,48
Mg_Message_count,7-Aug-12,42
Mg_Message_count,8-Aug-12,0
Mg_Message_count,9-Aug-12,0
Mg_Message_count,10-Aug-12,0
Mg_Message_count,11-Aug-12,0
Mg_Message_count,12-Aug-12,0
Mg_Message_count,13-Aug-12,0
Mg_Message_count,14-Aug-12,0
Mg_Message_count,15-Aug-12,0
Mg_Message_count,16-Aug-12,0
Mg_Message_count,17-Aug-12,0
Mg_Message_count,18-Aug-12,0
Mg_Message_count,19-Aug-12,0
Mg_Message_count,20-Aug-12,24
Mg_Message_count,21-Aug-12,24
Mg_Message_count,22-Aug-12,24
Mg_Message_count,23-Aug-12,24
Mg_Message_count,24-Aug-12,24
Mg_Message_count,25-Aug-12,24