I am running a multiple linear regression analysis to show how different demand factors affect demand of juice. My independent variables are price, temperature, competitor price, feature space and competitor feature space. However, I know there is an element of seasonality which is affecting the demand. I am looking at 2 years worth of weekly data, so feel that including dummy variables isn’t an option. I have seasonality indices which have been loosely worked out using an average of 4 years, removing sales caused by promotion and giving a percentage increase or decrease from how much sales vary on average from the base sale. How can I incorporate seasonality into my regression model?