Learn English – Origin of “You made the day!”

I found a print reference of the phrase as early as 1948 in the International Stereotypers' and Electrotypers' Union Journal, Volume 43:

Boy!, Oh Boy! I said I asked for a headache when I volunteered for this job, and I kid you not when I repeat it.

But it seems it was given a much wider audience as a phrase spoken several times by Lieutenant Commander Philip Francis Queeg in Herman Wouk's 1951 Pulitzer Prize winning novel, The Caine Mutiny:

That's the Navy for you. Pass the buck and get a receipt. Act at discretion, hey? Well that's exactly what I'm going to do, and I kid you not.

One of the colloquial definitions of grind is (from the Oxford English Dictionary):

Steady hard work; labour of a monotonous kind, esp. close and hard study; an instance of this, a dull and laborious task.

The first written use of this is:

1851 B. H. Hall Coll. College Words, Grind, an exaction; an oppressive action. Students speak of a very long lesson which they are required to learn, or of anything which is very unpleasant or difficult to perform as a grind.

So the grind of one's day would be the daily, monotonous work that one has to do. It is more common to refer to this as the daily grind.