Yes, I would say they can require you attend.

Unless you are salaried, you should also be paid for these hours. 
Paid Overtime if non-exempt or however your company policy reads if you are an  exempt Professional.

Nobody I know would like it, but sometimes companies need to use Saturdays when phones are not ringing and clients are not expecting you to reply asap.

I suppose it is safe to say that companies that require any mandatory training sessions on the weekends are not winning any Best Firms to Work For Contests, unless of course training involves some food and spirits at the local watering hole.  Most people work their tails off all week, putting in extra hours without pay, that asking them to come in on a weekend is no good in my eyes.  Phones ring, clients call, etc, I understand those distractions during the week.  So buckle down in a conference room and put all calls to voicemail unless an emergency.  Maybe try staggering the training sessions so that some of your employees are available to service your clients.  If you have no other choice to pull a session on a Saturday, then give your folks some comp time or send them out bowling or out to dinner with their family afterwards, on the company tab.

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