When planning out PR Strategy, editorial calenders, a list of the topics a magazine will be focused on throughout the year, may have been created for advertisers, but are a powerful PR tool as well. Editorial calendars, or "ed-cals" if you are in the know, are usually located online in the advertiser section, and sometimes will come as part of the media kit.
Quick example: If you have an accessory client who makes amazing bracelets, and see that in January of fashion glossy X that they are doing a story on accessories including clutch bags and cuff bracelets, you know to pitch your client for the January issue. If however, if you didn't look at your ed-cal sheet until February, you may have missed the window - and will look less than professional to an exasperated editor who tells you, "we just did a big feature on cuffs last month!" that is, if you can get the editor to speak to you at all.
Cision has released a new Executive Tip Sheet called "Ten Tips for Effective Media Planning and Editorial Calendar Research: How to Transform Your Editorial Calendar Collection into a Strategic Media Plan." Download it for free.
For more tips on ed-cals, Publicity-Insider has written up the basics.