Shivaratri is an auspicious day for Lord Shiva devotees as it marks the convergence of Shiva and Shakti. 

It is observed every month on the 14th day of the dark fortnight or the Krishna Paksha Chaturdashi Tithi.
The literal meaning of Shivaratri is the Night of Shiva.
In a year there are usually twelve Shivaratri days.
Shivaratri which falls during Shravana month is known as Sawan Shivaratri
This month, Masik Shivaratri will fall on Friday, August 06, 2021 in Vikrama Samvata 2077-78, as per the Hindu calendar. It will be also known as Sawan Shivaratri. The devotees observe a day-long fast to appease Lord Shiva and seek his blessings. 
As whole Shravana month is dedicated to perform Shiva Puja, which is considered highly auspicious.
Chaturdashi Tithi Begins - 18:28 on Aug 06, 2021 

Chaturdashi Tithi Ends - 19:11 on Aug 07, 2021 

Nishita Kaal Puja Time - 00:06 to 00:48, Aug 07 

Duration - 00 Hours 43 Mins 

The Masik Shivaratri puja is performed at midnight which is also known as Nishita Kaal. 
It begins with performing ‘Abhishek’ for Lord Shiva Idol or Shiva Lingam. Devotees offer Gangajal, milk, curds, ghee, honey, vermillion, turmeric powder, rose water, and bel leaves. 

Mantra you can chant: 

ॐ namah shivaya 

It’s the most auspicious day to worship Lord Shiva and seek his eternal blessings for inner peace. It is believed that fasting on this day blesses one’s soul with salvation or Moksha.