Strolling down the Mall to Buckingham Palace and watching the "Changing the Guard" ceremony is one of the must-do events in every tourist's stay.
There are actually two detachments of the Guard that need changing -- the one at Buckingham Palace and one at St. James's Palace, a short walk down The Mall. The St. James's Palace detachment starts first, by marching to the Palace at 11.13 AM (a few will remain behind, until they are replaced by the New Guard). They will then form up with the Buckingham Palace detachment on the forecourt. Meanwhile, the New Guard will march from Wellington Barracks in Birdcage Walk at 11.27 AM, ready for the ceremony to start at 11.30 AM. (If you want to, you can go and stand outside the iron railings of Wellington Barracks before the ceremony starts, and watch the new guard being put through their paces.)
The ceremonial change-over will then take place on Buckingham Palace forecourt, and the replacements will head towards St. James's Palace at 11.40 AM -- which then frees up the contingent that was left behind to head to Buckingham Palace at 11.55 AM. Finally, the Old Guard will leave the Palace at 12.10 and head back to Wellington Barracks, followed by the St. James's Palace detachment at 12.15. Phew! The whole event is accompanied by a military band.
If you want to bag yourself a spot right up against the Palace's railings, then we recommend getting there no later than 10 AM. If you find that it is already full, then try and get a spot on the Queen Victoria Memorial.
Be aware that the schedule is set by the British Army, and may change at short notice. The event is also heavily dependant on the weather.
[Note: You might also see some other soldiers marching down The Mall, but these are taking part in a similar ceremony at Horse Guards.]