You can find an overview of all the key dates for Edexcel AEA, GCE, GCSE, ELC and DiDA qualifications in chapter 2 of the UK information manual. 

Key dates for international qualifications can be found in chapter 3 of the international information manual.  Both of these chapters are available for download from this page if you prefer to view them online. 

Another useful tool is the online key dates calendar for general qualifications that has been devised by the National Assessment Agency (NAA) in collaboration with the unitary awarding bodies. 

This calendar, at www.modernisationonline.com/compkeydates/, gives an overview of administrative deadlines and other key dates (including an examinations timetable) for the UK awarding bodies AQA, Edexcel, OCR, WJEC and CCEA and common dates issued through JCQ.