In our times, the day of Rama-Janma is celebrated sometime during March-April.
For example, Rama-Janma (Chaitra Shuddha 9) Rama-Navami, occurred on 8 April (Yesterday) this year (2014).
In 2016, it will be on 15 April
In 2015, it will be on 28 March
In 2014, it was on 8 April
In 2013, it was on 20 April
In 2012, it was on 1 April
In 2011, it was on 12 April
In 2010, it was on 24 March
In 2009, it was on 3 April
Did you ever wonder why such variation occurs each year?
The answer is that a ‘Tithi’ (Lunar day) – i.e. same tithi (e.g. Chaitra Shuddha 9) occurs early (with respect to Western calendar – Gregorian Calendar) by ~10-11 days every year.
This is because while length of solar year is about 365 days, length of lunar year is about 354 days.
In order to synchronize solar year with the lunar year, an extra month – intercalary lunar month is added on an average every 2.8 years.
Thus for example.. let’s look at dates of Rama Navami above….
Last year, in 2013, Rama-Navami was on 20 April
This year (in 2014) it was on 8 April (receded by ~12 days)
Next year (in 2015), it will be on 28 March (will recede by ~11 days)
In 2016, it will occur on 15 April.
This jump of date to 15 April, in 2016 should tell us that an intercalary lunar month occurred (was inserted) sometime during 2015-2016. And the cycle would continue.
This addition of Adhika-Masa (intercalary month) indeed synchronizes luanr year with solar year and thus maintains semblance between the seasons and lunar months. (Note, in Gregorian calendar, seasons are fixed with respect to Gregorian months.. at least for a long time).
However, this insertion of intercalary month has no effect whatsoever, when we look at a time period exceeding say >4000 years. At that time (in future or past) we will indeed see a change in season with resepct to a given lunar month.
On an average, a season would shift by one luanar month every ~2000 years. Thus the change of season with resepct to luanar month is a very slow process and thus difficult to notice. It is made further difficult by periodic (~ every 2.8 years) insertion of Adhika-masa.
This happens to the phenomenon of ‘precession of Equinoxes’. Although this phenomenon is slow to detect, it is impossible to ignore after elapsing of long period of time.
For example, if we go back to say 12000 BCE (~14000 years in antiquity), the season of spring occurred during lunar months of Ashwin-Kartik. In our times, luanar months of Ashwin and Kartika corresponds to mid to late Sharad (pre-autumn).
During this ~12000 BCE, season of Sharad (pre-autumn) occurred during the lunar months of Chaitra-Vaishakha.
In effect seasons were reversed…as far as their alignment with lunar months was concerned.
e.g. Lunar months of Magha and Phalguna were months of rainy season while lunar months of Sharavan-Bhadrapada were months of Shishir (winter) season.