we celebrate Makar Sankranti. It is the only Indian festival celebrated on a fixed calendric day of the solar calendar. All other Indian festivals are celebrated as per the lunar calendar, which make their days of celebration on the solar calendar vary every year.
The difference is easy to see. In India, we follow a lunar calendar; the moon goes from new moon to new moon or full moon to full moon in 29.5 days. We get 12 full moons in 354 days, making a lunar calendar year 354 days long. However, the Sun returns to the same spot in the sky every 365.25 days. So, there is a difference of 11.25 days between the solar and lunar years. Every 2.5 years, therefore, an intercalary month (the Adhik Maas) is added to the lunar calendar to roughly synchronise the two.
This is crucial because weather patterns follow the solar calendar, not the lunar. On the other hand, accurate ‘mahurat’ (or ‘muhurat’) calculations are better done with the relatively faster moving moon. In fact, to make such calculations more accurate, the path of the moon, which is slightly off from the path of the sun, is divided into 27 ‘nakshatras’ while the path of the sun is divided into 12 ‘rashis’.