Alcohol while breastfeeding

Devon
So I've been EBF for 3months now. My birthday is coming up in July and I have my hen do in August, I still plan on breastfeeding at this point but want to have a drink and enjoy myself at these events. Thing is I don't really know the rules on how long after drinking I have to pump and throw the milk away? If we used formula rather than expressed milk for these occasions is my LO likely to reject my breast after because she might prefer formula?? Help advice needed!!!