Around your birthday begins a new solar cycle.
Sun reaching the exact degree of your natal Sun is your solar return.
Activates a particular house in your chart.
Ex: When one completes 32 years of age and enters the 33rd Sun's (soul) cycle it activates the 9th house of luck.

How to calculate which house is active in your chart?
Age minus multiple of 12.
Example: Someone born November on 28, 1990 is completing 32 tomorrow and entering the 33rd cycle of their life.
33-24: 9
9th house is active. Also called profection year in astrology.

Age : house active
0 to 1: 1st house
1+ : 2nd house
2+: 3rd house
3+: 4th house
4+: 5th house
5+: 6th house
6+: 7th house
7+: 8th house
8+: 9th house
9+: 10th house
10+: 11th house
11+: 12th house
12+: 1st house
13+: 2nd house
14+: 3rd house
15+: 4th house
16+: 5th house
So on.

This is why even our birthday is a good day for manifestation as well, setting intentions for ourselves, Making A Wish..
Solar Return chart (Varsh phala chart in Vedic Astrology) is also insightful for the year ahead.
Should be connected with current mahadasha, overall chart.