Prince Philip, Duke of Edinburgh opted out for his state funeral.
Thus, an 8-day mourning period was issued instead in the United Kingdom.
I can’t figure out why that period was determined exactly in eight days (and not, say, seven as in a week, six as half a dozen, twelve, ten etc.).
I looked at Wikipedia:ChristianBurial, but unfortunately it shed no further light on the number of days that Christian tradition allows from death to burial. NewAdvent.org makes the case of “keeping the corpses of important persons for seven days” before performing the burial; it’s sorta clue, IMHO.