Tulasi Vivaha (marriage of Basil tree) is considered to be a very important festival and also marks the beginning of the Marriage season amongst the Hindus. Tulasi Vivaha is the marriage of Vishnu (as Shaligram) with Tulasi, the Basil (also known as the Holy Basil) tree.
Once upon a time Tulasi Devi was married to a demon by the name of Jalandhara (one who was born from water). Jalandhara derived all his strength and power from her chastity and purity. So strong was this aspect of Tulasi Devi that even Lord Shiva could not defeat Jalandhara, when he declared war over the gods, by claiming sovereignty over the oceans and all the treasures that were churned out of the ocean during the samudra manthana. As it happens always, the gods went to Lord Vishnu for help. Vishnu decided to trick Tulasi Devi by assuming the form of Jalandhara and went to her. Tulasi Devi thinking Vishnu was her husband greeted him and her chastity was broken for a short while. Taking advantage of this, the gods managed to kill Jalandhara.
When Tulasi Devi came to know about this deceit, she cursed Vishnu to be a stone for being so stone-hearted. Vishnu accepted the curse and promised her that he would take the form of Shaligram shila on the banks of the river Gandika (now in Nepal). He also blessed Tulasi Devi, that she would reside with him at his abode, Vaikuntha, and on earth she would be seen as the purest of all pure things, and thus Tulasi plant is considered to be the purest of all, and a leaf of the plant is enough if put on anything to make it pure too. Tulasi is considered to be so close to Vishnu that the devotees never offer him anything without Tulasi leaves on it. On this day of Tulasi Vivaha, Lord Vishnu in the form of the black Shalgram shila gets married to Tulasi, as a plant on earth.
Besides this, there are few other myths which come to the similar conclusion of the annual marriage between Lord Vishnu (and sometimes Lord Krishna) and Tulasi plant. It is worth noting, that in the olden days, such traditions could have been initiated to grant an elevated position to the plant. This could be in recognition of its medicinal qualities. Tulasi or Basil is used for a number of common ailments, like skin disorders, coughs and colds, to name just a few. Recently during the outbreak of Swineflu, many doctors suggested a daily intake of 4-5 Basil leaves, to help strengthen ones immune system. Such festivals gave the plant an upgraded status and influenced people to plant them more commonly at homes, thus have the plant handy! It is not uncommon to find Tulasi plant in Indian homes for both religious and medicinal purposes. Thus, behind every ritual, there is a meaning or purpose. However, it is to each, as to how s/he celebrates such festivals. Grandeur and opulence in celebration might not have been in mind with our early thinkers, but since the advocates of religion have turned recognition to reverence – then so be it!