Ayanam can be DakshinAyan or UttarAyan.  = Winter and Summer Solstices.

Dakshin Ayan is when Sun is moving to South from June 22 to Dec 21 and 
Uttar     Ayan is when Sun is moving to North from Dec 22 to Jun 21.

Ayanan means movement. These two mean the Sun's movement to North or South. The correct term is The sun's declination to North or South.

This means the sun is South or North declined. Since the earth's axis is tilted 23 degrees, for 6 months, the sun appears more to the north and rest of the 6 months the sun appears more to the south.  This is why we have winter in northern hemisphere and summer in southern hemisphere for 6 months and then vice versa. This is why it is cold in Australia and hot in India for 6 months and then vice versa.

Sun reaches maximum South declination on 21 Dec and North on 21 Jun. These are called Winter and Summer Solstices respectively. These dates may be only 1 or 2 days before or later every year as our English calendar is based on Sun's position as earth revolves around it.