Because it's Saturday, that's the day most people have off and can get to the march. Most marches and demonstrations are on Saturday.
The 'honour' thing is two minutes silence at 11 o'clock observed wherever you happen to be.
It's tomorrow that the remembrance services and acts take place.
The march was two miles from the cenotaph.
Ok, but why THIS Saturday? After all, 7th October was a Saturday too. I didn't see 300,000 or so people in the streets of London protesting against Hamas atrocities ...