Ganesh Chaturthi is an annual festival celebrated with equal fervour in homes and together at a community level.
21 types of leaves (Patri) are one of the various offerings to the Lord on this auspicious day.
Though Tulsi is considered to be a holy plant, Tulsi leaves are never offered to Ganapati except during the 'pranpratishtha pooja' on Ganesh Chaturthi day (to 'breathe life into the idol'). There is an interesting story behind this. The following is the story available on the Shree Siddhivinayak Temple website....
Tulsi, the daughter of Dharmaraja. the God of righteousness, was roaming about in the world in ecstatic devotion to Lord Narayana, in her youthful days White thus going about, she came upon the banks of the sacred river Ganga. There she wandered into a serene ashram full of fragrant flowers and plants swaying in cool breeze. She saw Ganeshji, who was in the prime of his youth, beautifully dressed in yellow silks, and lost in deep contemplation on Lord Krishna. She was captivated by the beauty of Ganeshji, and entreated him to marry her.
Ganeshji turned down the proposal, saying that he was least interested in married life. Tulsi became angry and cursed Ganeshji saying : "You shall surely marry !", whereupon Ganeshji also pronounced a counter—curse on her : "You too shall get married, but to an Asura, and thereafter you shall be cursed to be born as a plant by the wise men". Realising her mistake, Tulsi pleaded for mercy, and extolled Ganeshji with divine hymns.
Ganeshji relented and said: "Though you shall be born as a plant, you will inherit the essence of all fragrance. All the gods will be happy with your fragrance, and Lord Hari will be specially glad to receive worship with your leaves. But you shall not be acceptable in the worship offered to me". So saying, Ganeshji left that place and went to Badrikashram.