On yahoo's stock quote page for a given share there's a link to key statistics, e.g. for vodafone. The calendar gives 1st half year profit at 14/11 (yesterday!) and annual report 30/6/06. Might be able to find the info you're looking for some where there?
Try the Investor Relations section of the company website - most companies have an event calendar. For smaller companies this may not be available, in that case just ring them up and ask someone in the Investor Relations dept.
Using the yahoo financial site is good. http://finance.yahoo.com/. Enter the ticker symbol for the company and select the headlines. Note that the majority of earnings dates fall on the same date every quarter (or within a couple of days) so use this can be used as a rough guide. The company usually announces the official earnings date/time a couple of weeks beforehand. I have found that www.earnings.com oftern have a date in for earnings for a company even though it is not official. They do however update the date when it is made official. Also check the investor relations section of a company’s website. Note that on the company website there is often an alert service which when activated will send an e-mail to you when a company PR or announcement is made. This keeps you in touch when if you stop browsing the net for a spell.