There are two ways of granting access to an object to every single profile in organization.
The first way is useful for Salesforce Administrators since this way doesn't need to write any code.
This is the way suggested by Guy here and Himanshu Parashar in this topic.
Check if you have Enhanced Profile List View Feature enabled. For this, go to Setup\Customize\User Interface under Setup header

Then go to Profile List View and Create a new View and add CRUD permissions for the object you need. You need to input the object name into search edit box and select all six CRUD setting for given object and click add button to include them in selected settings and then click save to save the view.

Then you need to check 'Select All' checkbox on the topmost-left corner of the view, then check "Modify All" and "Create". For each setting you need to select "All selected records' radio option and hit save.

And then access should be granted to all profiles. Note then for some profiles checkbox may still be unchecked, these are standard profiles and we can't modify those options.
Another way is for Salesforce Developers.
You can just open Developer Console and Execute such code there:
public static void grantAccess( String sObjectName ) {
// Strange but when I try to delete old records it raises internal salesforce error, but it works anyway even if we don't delete the previous records
//Database.delete( [ select id from ObjectPermissions where Parent.IsOwnedByProfile = true and SobjectType = :sObjectName ], false );
List<ObjectPermissions > os = new List<ObjectPermissions >();
List<PermissionSet> ps = [ select id from PermissionSet where IsOwnedByProfile = true];
for (PermissionSet p:ps) {
os.add( new ObjectPermissions(
ParentId = p.Id,
SobjectType = sObjectName,
PermissionsCreate = true,
PermissionsDelete = true,
PermissionsEdit = true,
PermissionsModifyAllRecords = true,
PermissionsRead = true,
PermissionsViewAllRecords = true
));
}
Database.insert(os, false);
}
grantAccess('Month__c');
Since we can't detect if Salesforce profile is custom or standard, we need to put second parameter as false to allow partial success in the code.