To display header in ALV grid use the following steps.
Build the ALV header table.
Pass the TOP-OF-EVENT subroutine name and callback program name to 

I_CALLBACK_TOP_OF_PAGE and I_CALLBACK_PROGRAM parameters of REUSE_ALV_GRID_DISPLAY function module.

Display the ALV header by passing the ALV header table built in   step one to function module ‘REUSE_ALV_COMMENTARY_WRITE’ inside TOP-OF-EVENT subroutine.

 
TYPE-POOLS: slis.
*----------------------------------------------------------------------*
*     Data Decalaration
*----------------------------------------------------------------------*
DATA: it_spfli TYPE TABLE OF spfli.
DATA: g_repid TYPE sy-repid.
DATA: it_listheader TYPE slis_t_listheader,
      wa_listheader TYPE slis_listheader.
*----------------------------------------------------------------------*
*     START-OF-SELECTION
*----------------------------------------------------------------------*
START-OF-SELECTION.
  g_repid = sy-repid.

  SELECT * FROM spfli INTO TABLE it_spfli.

  PERFORM build_alv_header.

  CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
    EXPORTING
      i_callback_program     = g_repid
      i_callback_top_of_page = 'TOP_OF_PAGE'
      i_structure_name       = 'SPFLI'
    TABLES
      t_outtab               = it_spfli.

*&---------------------------------------------------------------------*
*&      Form  BUILD_ALV_HEADER
*&---------------------------------------------------------------------*
FORM build_alv_header .

*  Type H is used to display headers i.e. big font
  wa_listheader-typ  = 'H'.
  wa_listheader-info ='Flight Details'.
  APPEND wa_listheader TO it_listheader.
  CLEAR wa_listheader.

*  Type S is used to display key and value pairs
  wa_listheader-typ = 'S'.
  wa_listheader-key = 'Date :' .
  CONCATENATE  sy-datum+6(2)
               sy-datum+4(2)
               sy-datum(4)
               INTO wa_listheader-info
               SEPARATED BY '/'.
  APPEND wa_listheader TO it_listheader.
  CLEAR wa_listheader.

*  Type A is used to display italic font
  wa_listheader-typ = 'A'.
  wa_listheader-info ='SAP ALV Report'.
  APPEND wa_listheader TO it_listheader.
  CLEAR wa_listheader.
ENDFORM.                    " BUILD_ALV_HEADER
*&---------------------------------------------------------------------*
*&      Form  top_of_page
*&---------------------------------------------------------------------*
FORM top_of_page.

  CALL FUNCTION 'REUSE_ALV_COMMENTARY_WRITE'
    EXPORTING
      it_list_commentary = it_listheader.

ENDFORM.                    "top_of_page
 
 
Serkan AKKAVAK
Computer Engineer
ABAP Developer & SAP MM SD Consultant