Strange that you mentioned this. I just wrote a post with an explanation of this. You can find it in my signature, or if you wana bookmark and read it later: http://www.novovo.net/2006/11/03/traffic-traffic-and-traffic/
http://problogwriters.com/index.php?option=com_smf&Itemid=34&topic=6.0 I also answered it in the only faq in my forum. I participate in several and my sites DO spike in traffic on post days.
Its a bit of a coincidence you have mentioned blog carnivals, because until yesterday i had never heard of them until i read about them here, http://www.problogger.net/archives/2006/11/04/introduction-to-blog-carnivals/ i might have a look into them and give them a go and hopefully get my blog more readers.
If anyone is still interested in blog carnivals - i got an urge to explain it in a bit more detail: http://www.novovo.net/2006/11/07/blog-carnival/ Enjoy.