How to test impact of certain event on dependent variable?

I know that we could use linear regression to find impact of one independent variable on dependent variable and calculate its $$r^2$$.

What if this event is not continuous and happens only once and want to calculate its impact. How can I do this?

For example: say Uber is giving out $10 worth coupon to every users on Valentine's day. I want to find its impact: • did it gather new users? • did it gather users who were not recently using uber to use it regularly? I can get result however there will be a lot of other variables that affect above numbers, how can I measure strength of$10 coupon on valentine's day?