You are wrong; Catholics do know. The Feast of Her Assumption into Heaven will be celebrated on the 15th of this month.
I'm sorry you don't "know", however.
No....Roman Catholics don't know....they think they do but don't.
Regarding the day, year, and manner of Our Lady's death, nothing certain is known.
The earliest known literary reference to the Assumption is found in the Greek work De Obitu S. Dominae. Catholic faith, however, has always derived our knowledge of the mystery from Apostolic Tradition. Epiphanius (d. 403) acknowledged that he knew nothing definite about it (Haer., lxxix, 11).
The dates assigned for it vary between three and fifteen years after Christ's Ascension. Two cities claim to be the place of her departure: Jerusalem and Ephesus. Common consent favours Jerusalem, where her tomb is shown; but some argue in favour of Ephesus. The first six centuries did not know of the tomb of Mary at Jerusalem.
Regarding the origin of the feast we are also uncertain.
It is more probably the anniversary of the dedication of some church than the actual anniversary of Our Lady's death.
That it originated at the time of the Council of Ephesus, or that St. Damasus introduced it in Rome is only a hypothesis.
http://www.newadvent.org/cathen/02006b.htm