By allowing Google and other search engine spiders to follow the links in your calendar might lead to problems with huge numbers of links getting indexed.
There is no reason to allow spiders to follow the links to the next and previous months.
Simply add the rel="nofollow" tag in the relevant places in the following files.
/includes/cal/
Example:
Change
<a href="<?php echo CAL_XHTML_Url($this->data['nav']['prev_link']);?>" title="<?php echo CAL_getResource('PREV_CALENDAR_TITLE');?>"><?php echo CAL_getResource('PREV_CALENDAR');?></a>
To:
<a href="<?php echo CAL_XHTML_Url($this->data['nav']['prev_link']);?>" rel="nofollow" title="<?php echo CAL_getResource('PREV_CALENDAR_TITLE');?>"><?php echo CAL_getResource('PREV_CALENDAR');?></a>