You are right; you needn't remind your students as they ought to be responsibly knowledgeable of all requisite dates. However, your propensity to offer just such a reminder may be supererogatory display of courtesy, and nothing to demur.
I'm guessing you do it because deep down you're an old softie who can't bear to see anything -- even themselves -- stand in the way of his students' academic success.
Personally, I think the people who don't hand in their assignments on time are probably the ones without enough common sense to check your blog regularly.
You're all right.