In align environment, how can you label some (but not all) of the equations?

Use the command \notag

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
x+y 
\notag\\
x-y 
\\
xy
\end{align}
\end{document}

EDIT: You can also use the command \nonumber (see comment)

EDIT: If you want to tag only one equation you can use \tag. In the following example I combine the command \tag with \refstepcounter to create the command \tageq

\documentclass{article}
\usepackage{amsmath}
\newcommand*\tageq{\refstepcounter{equation}\tag{\theequation}}
\begin{document}
\begin{align*}
x+y 
\\
x-y \tageq\label{1}
\\
xy
\end{align*}

\ref{1}
\end{document}