Makar Sankranti 14 January is when Change of Ayan occurs. North pole starts to tilt back towards sun. This goes on for the next 6 months.
What is its significance?
It is the onset on new ayan ( 6 month period when pole is tilting towards or away from sun). Makar sankranti is when sun goes uttarayan. This means the uttar or north pole starts moving towards sun. This 6 month period brings spring prosperity and brings the nature to life.
Any charity done on the day will give result for the next 6 months. Be sure to perform penance and give away things in charity on the makar sankranti to get good results.