Blogs are displayed in reverse chronological order; the most recent items are displayed at the top. Some WordPress.com users want a Journal-style site, which starts with the oldest post and continues in chronological order. Some people viewing a blog would like to be able to view in this order.
Displaying posts in reverse order is easy, just append the string ?order=asc to the blog URL, for example:
Writing a journal site
For people wanting a journal site there is no way to make the blog front-page display posts in chronological order. The best option is probably to have a static front page, which has a link to the reverse order page. This can be a simple “front cover” page with nice graphics and an “enter here” link, or include a synopsis or an index. The archives shortcode can be used easily to produce a basic index, or the more complex display-posts shortcode can give you more control over the result.