Some retailers see the trade uplift from towards the end of October, whilst others don't see that until December.
Planning takes many forms and needs to cover many areas-
Generally retailers will hire temporary staff to assist with the uplift in trade. For me, it's all about looking at your needs from the outset. Not just what hours you want to be covered but the exact nature of the duties you want these temporary staff to carry out. Think about the areas where you will see an uplift in sales, where your existing team will be best used and then you can identify any gaps in your weekly planner.
From this point you can plan extra hours x number of weeks x hourly rate to work out the costings of your extra Christmas staff.
When you are happy with the costings, it's about advertising the vacancy/vacancies whilst you design the recruitment process and timescales.
The fact that it's Christmas will be a part of the natural advertising for any retailer - people will arrive at your door looking to buy. You need to enhance this by your own marketing or advertising. This doesn't have to be expensive - changes to your shop window, your website and your social media presence can have a big impact.
Now I'm probably too late for Christmas 2014 but changes to your range in time for Christmas can and will make a difference to your sales. Gifting and capturing the imagination of someone buying for someone else has become a massive industry. If your range is too static to change this close to Christmas this year then ease consider what you may be able to do in time for next year.
Hope this helps.