Applying for part time job

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I have seen a job advertised for 15 hours a week in retail that I would like to apply for. However if for example it was only weekends or evenings it wouldn't work for me. There's nothing in the ad to say when or how the hours would be. What ought I to do, apply for the job and then find out how the hours work out or phone/email and ask first?

It could be any combination of weekdays,evenings and/or weekends and possibly more hours at peak periods (christmas, Sale). It is likely they have more than one position to fill if they are a large retailer in whcih case ask if you get an interview. If a small one it may be a specific role so you could call and ask.