It was decided by Bush Sr. over the summer. I think the idea was he wanted to bridge the divide now that Barbara was gone as she was the one who did not want Trump at her funeral.
As for McCain......there was such drama at that and way too much fanfare Imo......now every Senator will think they deserve likewise.
Bush-41 set the record for longevity among Presidents (although Jimmy Carter is likely to pass him in a few months). I hope Trump someday sets a new record. I hate to think how his funeral would be handled if there is a Democrat President at the time--national day of thanksgiving instead of a national day of mourning? They can't help themselves and it will no doubt get worse in the future.
The modern Democratic Party is what you get when you mix statism, fascism, and snowflakism.