Stutterers have, just like everything else really, good days and bad days. On a bad day every time you can stutter, you will stutter. Every phrase, letter, or sound combination that fumbles you up particularly (each stutterer is different remember) will. Normally, unless you have a severe case, you can get by with only fumbling maybe every other time you say it. Bad days will tongue tie you all day, all the time.

There are good days though. Days where I hardly, if ever even, stutter or get a mental block. These are good days indeed. Days where as a stutterer, I almost feel normal and fluent. Days where I forget that I even am a stutterer at all. 
I don't know how or when a good day will happen. It just does. Same as everyone else. No one can really predict when they'll have days where everything goes your way and days when everything is against you. Some days you just wake up and know: this'll be a good day.
Some days I wake up and know: This'll be a good day, I won't stutter much today.
While I certainly am glad that I have probably more good stuttering days than bad stuttering days. The inconsistency is difficult to navigate. I wish my stuttering patterns were more stable and steady. Just like I wish my speech over all was more uniform. If I knew I always stutter on "ch" or "th" words than I could just avoid those words all the time. But I don't know that. Sometimes I stutter on those words, sometimes I don't. It's only when I am about to say them that I know for sure.
Same is true for good stuttering days. I don't know when they are coming. I just know when they are here. But thank god they do show up.