The amount of tip you should give a hairdresser at Christmas time depends on the quality of their work and the relationship you have with them. A general guideline is to tip them at least 15% of the total cost of the service. However, if you feel that they provided exceptional service or you have a close relationship with them, you could consider tipping more. Additionally, if you're a regular client or you know they rely on tips to make a living, you may want to consider tipping a higher percentage.