Hi Anshika,
Try,
Declaring variable.
DATA: lv_field type c length 2,
lv_date type ekko-aedat.
IF lv_date+4(02) EQ '01'.
lv_field = 'st'.
ELSEIF lv_date+4(02) EQ '03'.
lv_field = 'rd'.
ELSE.
lv_field = 'th'.
ENDIF.
Hope it helpful,
Regards,
Venkat.