We'd like DH to go to a 4 day week. We're even happy for that 4 day week to in practise be about 35 hrs (he works very long hours at the moment)
We have a 4 yr old and Dh would like to spend more time with DS. Work is stressing him badly at the moment and family life is suffering a 4 day week would be much much better for us all - we can afford the 1/5 paycut OK in return for DH having friday at home!
So - good or bad time to ask? they frequently employ contractors on reduced hours and his work is project based so there's no reason he couldn't do his job on a 4 day week (he'd just drop one of his major projects).
I know you have the legal right to ask but there's no point if it will just get turned down and if it makes his boss see him negativly.