Christmas should be a time of joy, but for thousands of people sleeping rough, in tents or on public transport it will be anything but.
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Read it as "... it will be anything but that." (other than that/ except that -- it's no 'time of joy'). HTH. – Kris Sep 10 '19 at 7:16
"anything but" is a common idiom.
https://dictionary.cambridge.org/pl/dictionary/english/anything-but
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