Issue with AT SELECTION-SCREEN ON event

Hi Experts!!

In the below code, p_jan, p_feb are showing as blank though they are filled in. Is this how this event works?

PARAMETERS: p_categ TYPE zpycateg,
                         p_jan TYPE sy-datum,
                         p_feb TYPE sy-datum.

AT SELECTION-SCREEN ON p_categ.
IF NOT p_jan IS INITIAL.
***
ENDIF.

Please help me out.