Our Gemara on Amud Beis discusses the idea that certain funds are dedicated for one purpose and cannot be transferred to another purpose, no matter how noble. Thus, charitable funds donated for the indigent to use for the festive Purim meal cannot be used for other charitable needs. (The actual Halacha is subject to discussion and qualification, see Shulchan Aruch OC 694:2) 

Chavos Yair (responsum 232) rules that if a person gave someone food in honor of shabbos, he should not eat it during the week. (I presume, in our times with the advent of refrigerators and freezers, there is an assumption that when a person sends over food for Shabbos, there is implicit intention to enjoy the leftovers during the week.) Rav Yosef Engel in Gilyonei HaShas on this daf also compares this idea to a Gemara Yerushalmi (Nazir 5:1) that states if one pledges to bring a mincha offering for Shabbos, he is not permitted to bring that offering on a weekday. Apparently, even though it is the identical sacrifice, the dimension of bringing the sacrifice on a day which is characterized as more holy changes the performance of the mitzvah.

We see from this idea that the spiritual state of time (and possibly place) affects the mitzvah itself. How do we understand this? Intuitively it makes sense that certain times arouse human responses based on an accumulation of experiences and memories. It’s always a good time to buy a gift, but buying a gift on an anniversary is potentially more meaningful. It is not the mitzvah that changes, nor is it God, but as humans we are affected by the patterns of life. 

One of the great gifts of Judaism is in its ability to promote awareness of time. Aside from the annual cycle of holidays, rejoicing, grieving, and repenting, there are the cycles of purity and impurity that regulate marital life. As we saw in yesterday's blog post for Bava Metzia 77, when time stops, we notice.  There is a dread that comes into our awareness when we come into contact with death, we can either deny this dread with more distractions or reach for a higher part of ourselves that knows our soul and knows that this is our connection to the eternal.  The seasons and cycles of the Torah force us to constantly face the passing of time. A woman has her period every month and is reminded of the biological clock, but even the man must pause and assess his relationship, reflect on what needs to be renewed, caused by an enforced cessation of physical intimacy.  It is notable that the Jewish nation's introduction to the Torah begins with two mitzvos sensitive to time: Rosh Chodesh (Shemos 12:2) and the avoidance of the matzah dough fermenting and rising (Devarim 16:3).  The Torah reminds us that we have no time, life is short, and without the Torah we are a passing shadow (Tehillim 144:4). The greatest humans transcended their moments in time and lived outside of it by becoming attached to God and part of something bigger than the moment, though the moment must be seized. The great man sees the moment so he can seize the moment.