Is it ok to have a day off expressing ?

I normally express in mornings but it takes forever and I only get about 5 mls. I opened some formula yesterday which I did'nt use (Harry was cluster feeding to extent that boobs were sore and I was at the end of my thether but he fell asleep before needing it). So to give myself a break I am going to use this instead of EBM for today (I don't often use formula maybe once every 2 weeks). Anyway question is if I don't express occasionally will it affect my milk supply ? By the way Harry is 6 weeks