Most calendars are determined according to the position of the Earth in its revolution around the sun or the position of the sun in the celestial sphere and this calendar is the solar or in modern parlance the Gregorian calendar. The lunar calendar is based on lunar phases. The lunar year is the time taken by the moon to complete a revolution around the Earth and is generally just under 30 days. A solar year is roughly made of 12 lunar months or one lunar year. While a lunar calendar is easy to keep a track of, seasons would change every year. The solar calendar is constant. This is why most Hindu calendars actually operate as lunisolar calendars. The months reflect the lunar calendar but additional days/ months are added to make it synchronize with the solar calendar. A lunisolar calendar adjusts the discrepancies caused by trying to reconcile seasons with an accurate calculations of days, weeks or months.
Some lunisolar calendars calculate the days between two news moons and are known as Amanta calendars and some calculate between two full moons and are known as Purnimanta calendars. The Hindu calendar starts with the month of Chaitra, which generally falls between March and April. The dates keep changing due to lunisolar adjustments and also take into account sidereal dates. Sidereal basically measures the orbit of Earth around the sun taking a star as the basis. It reflects the time the sun is seen to leave one star and return to the same star along the ecliptic path and though it is roughly the same time taken for the Earth to orbit around the sun, the solar calendar is approximately twenty minutes slower than the sidereal. Using sidereal, gives astronomers some flexibility since stars are considered to be fixed unlike the motion of the Earth around the sun that keeps changing location possibly due to conditions like inertia and gravity. In everyday life, people use solar time and not sidereal time.
The Hindu lunisolar calendar is used to determine festivals and important occasions. Some communities celebrate festivals depending on the solar calendar. In these cases, the festival usually falls on the same date every year. The Hindu solar calendar begins with the Rashi, Aries or Mesha and ends at Pisces or Mina. The Hindu calendars differ from community to community. There are Tamil, Bengali, Orissa, Punjabi, Tulu, Kannada and Malayali calendars as well as the Vikram Samvat calendar and the official Indian Government adopted Hindu calendar known as the Saka calendar, among others.
Regardless of which Hindu calendar is being used, the names of calendar months are similar sounding since they are based on Sanskrit words. All Hindu calendar days start at sunrise and end at sunrise the next day. The Hindu calendar is known as Panchang because it is made of five essential parts or angas. These are tithi or lunar days, vara or the day of the week, nakshatra or the lunar houses, yoga or alignment and karana or half a tithi. There are many precise calculations required to calculate days, additional months, skipped days in a month etc. However, the above highly simplified explanation is a good rule of thumb to follow.