7-8 February 18

On 7 February 1863, English chemist John Newlands published one of the first table of elements, which divided the known 56 elements into 11 groups based on the "Law of Octaves." This suggested that any one element will have similar properties to elements eight places before and behind it on the table. (Source -- National Day Calendar.) This was six years before Dmitri Mendeleev published his more systematic table which became the foundation for what we use today. Mendeleev was born on 8 February 1834, so these two days are special times to honor the periodic table of the elements!