Significance of Mahashivratri vrat
Why do we celebrate mahashivratri? Why do we treat this day as an important festival?
According to some legends, Mahashivratri was the wedding day of Lord Shiva and Goddess Parvati and lord Shiva did the heavenly dance also it is believed that on this day Lord Shiva drank kalakuta or halahal vish which appeared from samudramanthan event.

As from the name itself we can understand, Mahashivratri-the great night of Shiva. This day comes once a year in late winter (February/March, or Phalguna) and before the arrival of Summer.
What happens when a unmarried girl or woman do fast on this day?
Lord Shiva is considered as a perfect husband. He is a family man, a perfect father and a perfect husband. If an unmarried girl or woman do mahashivratri vrat it is believed that she may get a idol and strong husband like lord Shiva .

People who want to do fast for Mahashivratri can only consume fruits, milk and recipes with fast-compliant ingredients. One have to say no to meat and consumption of tobacco or alcohol on this day.
To know, Why do we keep vrat on this day? What we will get when we keep mahashivratri vrat? How it all begins? let us go a long time back:
Once there was a man who never did any good sin. He kills animals especially deer for his food. He steals, he kills and did all kinds of bad sins.
On one day his family ask him to bring food as they all were very hungry. So he decided to go to jungle and kill deer for their food. He searches for deer's but couldn't find a single deer who he could and fulfil his and his family hunger.
He went tired and went to a lake and there was Aegle marmelos tree(Bael patra tree-Leaf's used for Hindu rituals especially for lord Shiva pooja). He sat on a branch of that tree with his arrow and bow ready hoping that if any deer arrives to drink water from that lake then he could kill that deer.

Note:- Pahar(or Prahar commonly pronounced peher, is a traditional unit of time used in Vedic India, Pakistan, Nepal and Bangladesh. One pahar nominally equals three hours, and there are eight pahars in a day)
After one Pahar a female deer came to drink water from the lake. The man saw that deer and while preparing his bow and arrow, accidently some bael patra and water from his kalash fell on a shivling located down the tree and his first prahar pooja done and some of his sins gets cleaned.
The deer hears the water noise and looked towards the tree and saw a man sitting on the tree. She asked him, Hey! What are you trying to do with that arrow? I was preparing to kill you and make you my food, he replied. She told him that I am very glad that my body will be useful for someone needy and I will get heaven after my death because of this sacrifice but for sometime please let me go and give my kids to my sister and secure their future, she added. The man refuses to let her go to her children as he don't believe her. She tries to convince the man that she never told any lie in her life and she promises the man that she will come to him soon. But man refuses. She give him a proposition that if she will not come back then she will lose result of all good deeds she had in her life. After listening to her promises he allowed her to go to her shelter for a while. She then went to her shelter.
After sometimes another female deer went to that lake to drink water. Actually she was the sister of the previous deer. She came searching for her sister as its been very long since her sister went out to drink water. As the man saw that second deer he prepares his bow and arrow to kill that deer and suddenly again some water from his kalash and beal patra falls from the tree on the shivling by mistake and second prahar pooja also done. Hearing the noises of water the deer saw the man on the tree. She asked him, What are you trying to do with that arrow? I was preparing to kill you and make you my food, he replied. She told him that I am very glad that my body will be useful for someone needy but please let me go to my shelter for sometime so that I will leave my children to my sister's husband, she added. The man refuses to let her go to her children as he don't believe her. She tries to convince the man that she never told any lie in her life and she promises the man that she will come to him soon. But man refuses. She promised him that if she will not come back to him then she will lose result of all good deeds she had in her life. After listening to her promises he allowed her to go to her shelter for a while. She then went to her shelter.

After some time a healthy and strong male deer came to the lake to drink water and man saw that deer. While aiming the arrow on the deer again some water from his kalash and some bael patra falls from the tree and third prahar pooja also done but accidently. The male deer saw the man sitting on the tree and asks him that what he is trying to do? The man replied, I am attempting to kill you from my arrow and I will feed my family. The deer said, it will be my great pleasure that my healthy body will be useful for someone needy and my soul will be blessed by god but let me go to my children and I will give them to my neibours and they will take care of them. The frustrated mam said him that great! you also go and you will never come like other promised and didn't appeared yet. You are also a big lier, he added. The deer told him that he had never told any single lie in his entire life but if you are not able to trust me yet then I promise you infront of lord Vishnu that if I didn't came soon then I will lose all good results which got collected through my good deeds. After doing many big promises the man allowed him to go to his children for sometime. So he also went to his shelter.
All three deer's went to their shelter and meet each other. Actually they all were related to each other and they found that they all were bounded by promises they did to that man. The female deer told others that she have to keep her promise as children needs their father and sister will take care of them. The sister told others that children needs mother and father not aunt so she should keep her promise. Then the male deer told them that the children needs their mother importantly as compared to father so he will sacrifice his life. But no one is ready to take other's life so they all went to that person. While hearing all their conversation the children went with their parents and aunt thinking that whatsoever will happen to their parents same should happen to them also because they cannot live alone without their parents.

All the deer's went to that man and after seeing all deer's the man aims his bow and awwor to kill all the deer's then again accidently the water and leaves fall from the tree on the Shivling and fourth prahar pooja was also done successfully. Now with this final prahar pooja all his sins gets cleared and he is now fully cleaned. The deer's told the man that as per our promises we now came to you to sacrifice our life's for you and your family and now you can kill us all and feed your family. Listening to their words the man realizes that being the animal they are so cultured and they have so good promise keeping nature but I am a human and I kills animals for my own profit and on the other hand those animals are sacrificing their valuable life's for me. He told all the deer's that they can go and live a good and long life I will not kill you now.
After this event lord Shiva came to the man and blesses him and gave him a name. The name given to the man was 'Guha'. Guha was the person who assists Rama (from the epic 'Ramayana') to cross the river during Rama's banishment from Ayodhya by his stepmother.

Hence we can conclude from the above story that If someone does the mahashivratri vrat properly then his all sins will be cleared and he will be blessed by lord shiva.
At last, Happy mahashivratri to all my readers, stay happy and blessed always. Thanks for reading.