Do it now or wait?

So I've made the decision to resign as co-director from my company. The wheels are in motion, I've already started changing some of the info, I have already typed up my letter, etc. Should I send it now or wait until month end?

My ex boss alerted me to the fact that my co-director is job hunting (she loves her job but we've been struggling financially (her actions don't help)). Anyway, I don't want to risk her resigning before me and then I'm stuck with a company where my name has been dragged through the mud due to her incompetence.

Should I do it now (well, maybe tomorrow or Friday as she's on a personal day)? Or wait until end month like I originally planned?