According to the link listed in a previous comment, it looks like you may need to use "tags" instead of "tag_id". If that doesn't work, the default for a WP_Query is "tag". Also, just to check your syntax, when you use that query you're not putting quotes around 20, are you?
post_type=ai1ec_event&post_status=publish&posts_per_page=6&tags=20
See if that helps