The Month of Kartik begins today. It's significance is mentioned in various Puranas. Kartik Month is the most favourite month of Shri Vishnu.

The Month of Kartik begins today. It's significance is mentioned in various Puranas. Kartik Month is the most favourite month of Shri Vishnu. The sin destroying month of Kartik is said to be endowed with divine influence.‪ As per Shrimad Bhagwatam, Kartik month is ever so dear to Lord Vishnu and is the bestower of both worldly pleasures and moksha.
In Skanda Purana, it's mentioned that Lord Brahma had revealed to Narad that 'Kartik' was supreme among all the months just as lord Vishnu is supreme among all the deities and Badrikashram is the crown of all the places of pilgrimage. According to Brahma all these three would have greater significance in Kaliyuga as compared to any other yuga. Kartik is very dear to Lord Vishnu and any virtuous deed done during this month never goes without being rewarded. One important characteristic of this month is that all the deities live in the proximity of human beings for the entire period of the month and accept everything that is offered to them. Penance done in this month or donations made during this month is believed to give Imperishable virtues. There is a special significance of donating food grains during this month. Similarly, worshipping Shaligram during this month also has great significance. Observing austerities during this month or worshipping lord Vishnu with appropriate rituals liberates one's ancestors from the tortures of the hell. Worshipping Lord Vishnu with petals of Lotus helps him to become absolved of all the sins committed in his previous many births. 
Worshipping lord Vishnu with tulsi leaves helps one in getting cured of incurable diseases. Because Tulsi was born on the lunar day of the full moon in the month of Kartika, Lord Narayana prescribed this day for her worship. Whoever worships her on this day and on the month of Kartik will be freed from all sins and go to Vaikuntha. Whoever, out of reverence, gives Lord Vishnu a Tulasi leaf in the month of Kartika, will gain the same benefit obtained by giving the gift of ten million cows. Among all articles which are offered to the Lord, tulasi is most highly praised. Therefore one may perform perfect worship of the Lord by simply offering pure water and tulsi leaves. 
According to the Garuda Purana, tulsi must always be offered in the naivedyam.
"Tulsi vina ya kriyate na puja, snanam na tat yat tulasivinakrtam
bhuktam na tat yat tulasi vinakrtam, pitam na tat yat tulasi vinakrtam" 
Means ~ Puja, bathing and offering of food and drink performed without tulasi cannot be considered as puja, bathing and offering to the Lord. The Lord does not accept anything or drink anything that is without Tulsi, as per Brihad Naradiya Purana
As per Padama Purana, one who worships lord Vishnu/ Krishna with Tulsi leaves attains the results of all kinds of austerity, charity, and sacrifice. In fact, he has no other duties to perform, and he has realized the essence of the scriptures. Padama purana states one who puts mud from the base of Tulsi tree on his body and worships the Deity of Lord Krishna gets the results of one hundred days worship each day! 
Om Namo Bhagwate Vasudevaya Namah ~ Om Namo Narayana ~ Jai Shri Vishnu!