But anyone who is taking on seasonal self-employed work this Christmas is being urged to ensure their taxes are in order. Tax experts are warning that those who fail to comply to HM Revenue and Customs (HMRC) rules on making money from side hustles could find themselves in hot water with fines and penalties. You should then file a tax return and pay any tax due by 31 January 2026. Any sales made this Christmas will fall into the 2025-26 tax year and need to be declared, and any tax on them paid, by the end of January 2027. HMRC has launched a Help for Hustles campaign to help traders understand when tax rules apply to them.