The date is conjectured from the relationship between this letter and the letter from John Cattell, 12 May 1860 (Correspondence vol. 8) and from the date of the first John Cattell’s death (see n. 2, below).
John Cattell (1786/7–1860) was a nurseryman in Westerham, Kent, with whom CD had done business for several years (see Correspondence vol. 5, letter from John Cattell, 13 August 1855). He died on 5 May 1860 (England & Wales, national probate calendar (index of wills and administrations), 1858–1966 (Ancestry.com, accessed 12 June 2015)), and his son, John Cattell (1819/20–69), took over the business (Florist, Fruitist, and Garden Miscellany (1861): 197). CD evidently wrote a letter of condolence that has not been found, but see Correspondence vol. 8, letter from John Cattell, 12 May 1860.