It seems that the week numbers could not be displayed in the horizontal and vertical line calendars. I added the following statements between the tags </today> and <weekdays> of horizontal line calendar.xml file in Shadow4, refresh rainlendar but still couldn't see the week numbers. I also tried to adjust the height and width of the calendar but it didn't help. Did I miss something or it was caused by a program bug?

The week numbers are not supported by the horizontal and vertical layouts.

Well, maybe you should update the weeknums page in the help files, which describes the weeknums as follows:
<weeknums>
Contains an appearance which draws the week numbers to the calendar. This is optional element. On GRID layout the week numbers are drawn on the left side of the days. On the HORIZONTAL and VERTICAL layout the week numbers are drawn behind the days but you can use the offset to move them to the desired position. The FREE layout doens't support week numbers.

This description suggests that weeknums is supported in the GRID, HORIZONTAL and VERTICAL layouts. Actually, my first thought was that the weeknums was not supported in the two layouts when I came to the question, because I couldn't imagine the display style of the week numbers...

BTW, the Rainlendar_SetEventHandler page should also be updated because the rows for values 14, 1000 and 1001 are not displayed correctly in the table to explain the possible values for the event parameter. The problem also exists in the API-reference.pdf file.

Ok, I had to check this from the code and the week numbers have indeed worked in the horizontal and vertical calendars before but for some reason I removed the support a while ago. I have no recollection why that was done. I'll have to dig a bit deeper and see if there was a real reason for this or if it was jus a mistake.