To me, sadhana is a daily spiritual practice allowing time and space for an individual to turn inward.
As Yogi Bhajan (of the Kundalini yoga tradition) says, "Sadhana is self-enrichment. It is not something which is done to please somebody or to gain something. Sadhana is a personal process in which you bring out your best."
Sadhana could be taking a walk in nature, doing breath work or yoga asanas on a mat, spending time meditating or chanting, reading and reflecting on a poem, or simply watching the sun rise.
Please accept this post as a possible starting point for your own practice today, and as a basis for your new year.
Mantra:
Lokah samastah sukhino bhavantu
Translated as:
May all beings, everywhere, be happy and free. And may the thoughts, words, and actions of my own life contribute, in some way, to that happiness and to that freedom for all.