According to the new ACL submission policy for submitting papers to preprint services such as Arxiv, there is a period that all ACL submissions are forbidden to be submitted to Arxiv. The period begins one month before the submission deadline and ends at the notification date. The changing does a lot of impact for NLP researchers. Some people just get used to posting their paper on Arxiv right after the submission; this is strictly forbidden now. If you want to submit to Arxiv, then do it one month before the deadline.
As the length of the period is about two months, which means that in this two months, NLP researchers can reduce the frequency of checking Arxiv papers, just like a holiday. However, it will also have a negative impact. For researchers also want to submit to ICLR, they have to carefully check the schedule of ACL submissions. It is possible that the forbidding period conflicts with the submission deadline of ICLR, then the researchers have to choose to submit to either conference.