Hindu Lunar Calendar divisions (Paksha, Masa)

Hindu Calendar (lunar calendar) month is divided into two "Pakshas", and 30 lunar days (Tithiyan). These lunar days are roughly equal to 29.5 solar (Roman Calendar) days. The first fortnight is called Shukla Paksha (Sudi) or the bright fortnight ... begins with the full moon. The dark fortnight or the Krishna Paksha (Badi) begins and ends with full moon.

Each Tithi or lunar day of the 15 phases of the moon has its special significance, and marks the appropriateness for a particular ceremony or observance.

As the 12 lunar months make up only 354 solar days, there is an additional month called "Adhika masa" after every 30 lunar months. This leap month is generally either Ashadha or Shravana, and is known as second (dvitiya) Ashadha or Shravana.

The Hindu Lunar Calendar begins with Chaitra (March-April).

(source: Pg 9, Fasts and Festivals of India, by Manish Verma, Diamond Pocket Books)

names of bright & dark fornights of a month in Hindu Calendar

Here are the names of the days (15 days) around the waxing and waning of the moon. This is as per Hindu Lunar Calendar.

1. Pratipada - 1st day of lunar fortnight
2. Dvitiya - 2nd day
3. Tritiya - 3rd day
4. Chaturthi - 4th day
5. Panchami - 5th day
6. Shasthi - 6th day
7. Saptami - 7th day
8. Ashthami - 8th day
9. Navami - 9th day
10. Dashami - 10th day
11. Ekadashi - 11th day
12. Dvadashi - 12th day
13. Triyodashi - 13th day
14. Chaturdashi - 14th day
15. Purnima - 15th day (Full moon day)
    
16. Amavasya - 16th day (no visible moon in the night sky)

(Source: Pg 10, Fasts and Festivals of India, Manish Verma, Diamond Pocket books)

Months of the Hindu Calendar

1. Chaitra (March - April)
2. Vaisakha (April - May)
3. Jyaishtha (May - June)
4. Ashadha (June - July)
5. Shravan (July - August)
6. Bhadrapada (August - September)
7. Ashvina (September - October)
8. Karthika (October - November)
9. Margshirsha/ Agrhayan (November - December)
10. Paush (December - January)
11. Magha (January - February)
12. Phalgum (February - March)

(Source: Pg 10, Fasts and Festivals of India, by Manish Verma, Diamond Pocket Books)

Ritus or seasons according to Hindu Almanac

According to the Hindu claendar there are six Ritus or seasons.

1. Vasanta Ritu (Spring season) - March-May
   
2. Greeshma Ritu (Summer season) - May-July
   
3. Varsha Ritu (Rainy season) - July - September
   
4. Sharad Ritu (Autumn season) - September - November
   
5. Hemanta Ritu (Winter season) - November - January
   
6. Shishir Ritu (Cool season) - January - March

(source: Pg 11, Fasts and Festivals of India by Manish Verma, Diamond Pocket books)