I was always under the impression the term "mishmar" started in Volozin where the yeshiva students wanted to make sure learning was taking place twenty four hours a day in the Beit Midrash and as such had "shifts" of learning. These shifts where called mishmar after the "mishmarot" in the Beit Hamkidash in which the families of Kohanim would rotate every week to do the avodah. Each family had their own week when they were on "watch" i.e. duty and so too in the Yeshiva groups of students had different shifts when they were on duty to ensure learning was taking place in the Beit Midrash. Thus, that is the connection to the word shomer which means to guard, watch, hold vigil etc.
Although I believe mishmarot took place in Volozin every night (I could be wrong) I believe the custom to adopt a Thursday night mishmar is due to the fact that Friday is often a day which is less conducive to full time learning as people need to get ready for Shabbos and as such people try to "make up" for this loss of time by learning extra on Thursday night. When I was growing up, doing a mishmar meant staying up all night Thursday night to learn, however in time it has been come to be used to connote any amount of extra learning done on Thursday night.
Personally, when I was in Yeshiva I never understood the concept of staying up all night on Thursday with an over tired brain and then sleeping all Friday morning (there were those who did that)....it always seemed to me to make more sense to sleep normally on Thursday night (perhaps learning an extra hour or so) and then learn properly on Friday...but I suppose that is a separate issue. :-)
Tzvi Daum
[
torahskills.org]
[
twitter.com]
