documentclass[a4paper]{article}
usepackage{geometry}
usepackage{longtable}

begin{document}
begin{longtable}{l}

longtable header: this is page thepage
endhead

A \
A \
A \
... however many rows ...
... needed to exceed one page ...

end{longtable}
end{document}

documentclass[a4paper]{article}
usepackage{geometry}
usepackage{longtable}
usepackage{everypage}

newcounter{mypage}
setcounter{mypage}{1}

makeatletter
AddEverypageHook{stepcounter{mypage}%
  ifdimhtLT@head>0pt
    dimen0=htLT@head
    globalsetboxLT@head=vbox todimen0{strut
      longtable header: this is page themypage}%
  fi
}
makeatother

begin{document}
begin{longtable}{l}
longtable header: this is page thepage
endhead
rule{0.7textwidth}{0.7textheight}\
rule{0.7textwidth}{0.7textheight}\
rule{0.7textwidth}{0.7textheight}
end{longtable}
end{document}