The beginning of spring is not always determined by fixed calendar dates. The phenological or ecological definition of spring relates to biological indicators; the blossoming of a range of plant species, and the activities of animals, or the special smell of soil that has reached the temperature for micro flora to flourish. It therefore varies according to the climate and according to the specific weather of a particular year. Most ecologists divide the year into six seasons that have no fixed dates. In addition to spring, ecological reckoning identifies an earlier separate prevernal (early or pre-spring) season between the hibernal (winter) and vernal (spring) seasons. This is a time when only the hardiest flowers like the crocus are in bloom, sometimes while there is still some snowcover on the ground. http://en.wikipedia.org/wiki/Spring_(season)
shukla(white&bright) paksham(one lunar phase or fortnight) means the waxing moon phase or the 15 days before the full moon. Krishna(dark) paksham means the waning moon phase