Twitter Hashtags serve two purposes – to categorize tweets and as a tracking mechanism to see how far a particular tweet and tweet theme spreads which can be measured using tools such as TweetReach. An effective hashtag should be as unique as possible.
You can quickly see the problem with non-unique hashtags. For example, various Oregon State and Ohio State Twitter accounts both use the hashtag #OSU, so their tweets are always categorized together, even though they shouldn’t be. While it’s not as short, I recommend that each university use a more unique hashtag. Perhaps #ORESTATE for Oregon State and #OHSTATE for Ohio State?
Consider this for your own tweets. If you want to track them later, using generic hashtags such as #monday isn’t going to help.