Organised by the Charitable Giving Association, it involved hundreds of people from all walks of life, beginning on December 25th and lasting for three days.
Just to close the loop on this, your sentence is fine as it is since it gets across your intended meaning and doesn't really break any grammar rules.
However, as a matter of style the adjectival phrase "beginning on ..." could benefit from being closer to the pronoun it modifies ("it"), as in the following example:
Beginning on December 25th and lasting three days, and organized by Charitable Giving Association, it involved ...
Or, alternately, if you really want to keep the date range at the end of the sentence (for emphasis or effect) then you can structure the sentence as a list of features:
... it involved hundreds of people from all walks of life, and began on December 25th and lasted for three days.
The double "and" here is awkward, though, but this is the kind of challenge all writers must wrestle with.
... it involved hundreds of people from all walks of life and lasted for three days, beginning on December 25th.
Still a bit awkward. Let's try again:
... it involved ... and lasted for three days, from December 25th to December 28th.
I'm not sure I like this any better, but you get the idea.
As a side note, if you're talking to an audience that recognizes the date, saying something began on "December 25th" is also awkward, since everyone should know that's Christmas Day. I assume the organizers of the event did this on purpose, so it's weird not to explicitly say, "It began on Christmas Day".