What it means is that not to forget why there on their jouney to get there bodies back,and its the 日期 when they tried to transmute their mom,thats why Winry was so suprised 由 what was in it.
It's the 日 Edward burns their house right after Edward get the tittle. It shows their determination of not turning back and 移动 on toward their journey in seeking the ways of returning back theirs body.
they explain it later on.. but yeah,it says "dont forget Oct. 3" and Ed wrote it on the 日 he and Al burned their house down. Ed claimed to have written it so that he would remeber not to ever turn back but his father pointed out to him that he really burned his house because Ed didnt want to be reminded.yeah..i'll shut up now... O.O
