Vriddhi means increment. In a Hindu calendar, a day and night together is called a tithi. Sometime the same tithi is found on two day in Hindu calendar and this is known as Vriddhi tithi. A lunar month has thirty tithis. The period require for the moon to go 12 amsha away from the sun is counted as one ithi.
If the moon’s orbit had been exactly circular, the moon would have taken an equal period to cover these 12 amsha every time. But it is not so. Moon’s orbit is elliptic round the earth.
When the moon is near the earth, its speed increases; when it is farthest from the earth, its speed decreases. Hence the time to cover 12 amsha increases or decreases. But the total period of all the tithis is 29.56 solar days. The difference of one day in a month is compensated by incrementing a tithi.
For example – If the 8th tithi (ashtami) commences on Thursday at sunrise and continues for 57.2 ghatikas that is up to early morning of Friday, then the 9th tithi (navami) starts on Friday and continues up to Saturday, 1.20 ghatika after sunrise of Saturday. Now, a tithi is reckoned at what it was at the time of sunrise. Hence navami is on Saturday also. This means that the navami tithi extends to Saturday also, hence the navami is affected by vriddhi.
A solar year is of 365.25 days where as lunar year is about 354 days. Hence there is a difference of about 11 days in a solar and lunar year. In three years, this difference is about 30 days, which amounts to one lunar month. In order to adjust the calendars, one lunar month is added in every third lunar year so that year has 13 months. This month is called an intercalary month (Adhika Masa). Hence that year has a vriddhi of one month.