In Article 1, Section 9, Congress is limited, particularly, from prohibiting the "Importation" of slaves, before 1808. The slave trade was a bone of argument for many, with some who supported slavery abhorring the slave trade.
The 1808 date (a compromise of 20 years) allowed the slave trade to continue, but placed a date-certain on its survival. Then eventually the Congress passed a law banning the slave trade that became effective on January 1, 1808.
Hope this helps ;)
