Dartmouth sends out approximately 500 likely letters in three waves starting in mid February. Remember that majority of students accepted during the RD round will not receive a likely letter. The letter states that your admission to Dartmouth is likely. They also send you information about Dimensions (admitted student days).
Personally, I have a love-hate relationship with the process (and yes, my kid received a likely letter)) as it is designed to put you at ease and not to worry because your admission to Dartmouth is likely. On the other hand I think the process creates more angst as student who do not receive a letter think that they will not get accepted.
We are now into March. While it has been a long admissions cycle, it will be over soon. I have every confidence that every one will land someplace great.