Holst is probably no longer under copyright in either the UK (75 years) or USA (50 years). Often it depends what country your in though.
In the UK, Sergei Rachmaninoff is still copy protected (dead less than 75 years), but likely not in the USA (dead for more than 50 years)
Other things are taken into consideration. Gershwin has been dead for 75 years this year - however his wife wrote lyrics for his compositions and so his works are, for the time being, still protected by copyright law in the UK.
We always contact the authorities to make sure before using any music thought to be out of copyright.