I assume that you mean the RBL event at the Royal Albert Hall. (There are other Festivals of Remembrance across the country, e.g. in Bristol).
The only web reference I can find to the whole event being transmitted (rather than just edited highlights) was when Radio 2 allocated a 3-hour slot to it, so your answer might well be 'around 5pm'.
However I suggest contacting the Royal Albert Hall for a definitive answer:
http://www.britishleg...embrancefactsheet.pdf
Chris