Refer to figure.
The ITCZ follows the sun in that the position varies seasonally.
It moves north in the Northern Hemisphere summer and south in the Northern Hemisphere winter.

Therefore, the ITCZ is responsible for the wet and dry seasons in the tropics.
The sun crosses the equator twice a year in March and September, and consequently makes for two wet seasons each year.
In December and July, when the sun is at its greatest extent north or south of the equator makes for two dry seasons.

Further away from the equator, the two wet seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season.
In the Northern Hemisphere the wet season occurs from May to July, in the Southern Hemisphere from November to February.