It doesn't really require any legal advice though really, what it comes down to is two things:
Is it a problem?
A lot of people (myself included) have had 16 hour a week contracts in retail and had to compete for more hours, so if you're not in college (assuming until sept?) then a few months of work means you will have a lot more money. If it were me, I wouldn't really be arsed about losing a few days in the summer, I'd get over it, you can make up for it by - for example - thinking that the extra money will buy an XB1 or PS4 before long. Or something else you could save for.
Secondly, showing that flexibility and commitment to help the store/boss out when they need you might just be a massive boost if you want any further opportunities there, or at least maybe if a reference is required. Getting ahead in life is down to the small details such as putting yourself out for an employer and, trust me, it can make a big difference.
The "can they" side..
For this, it will simply come down to your contract. If your contract (and you should have one, not necessarily a paper copy) states that you may be asked to work unavailable hours at managers discretion that you are required to work (for example, bank holidays), then yes they can.
But it depends on what it says in there. Perhaps there's some scaremongering, perhaps it is a fact, but never think that they wouldn't sack you when they're short staffed anyway as they might find people unwilling to work extra hours are disposable.
Hope that's of some use. The only barrier you will have, depending on age and what you signed, is the 50 hours a week side of things