Usually when we say "This is not my day" we mean that we are having a bad day. Things are not turning out as we had planned.
However, I have recently tried to say early in my day, "This is not my day" to remind myself that this is God's day, not mine. "This is the day which the LORD has made; let us rejoice and be glad in it" (Psalm 118:24). I should be seeking to fulfill His plans, not mine. The success or failure of my day should be benchmarked against how much my actions conform to His will.
Everything I have is God's, not mine. That includes my time. It is often the distractions and the obstacles that are where we are most closely encountering God in our day. When we appear to be most off course, most off track, it is precisely then that we are often most engaged in doing God's will.
Ironically, it is when I accept the fact that "This is not my day" that my day will be better, not worse.
St. John Bosco: Conquering Souls for Christ
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