Psalm 118:24 — This is the day which the  Lord hath made; we will rejoice and be glad in it.

For both Jews and Christians this much loved verse is explained as referring both to historic events, and to events in the prophetic future. To David it seems this verse might refer to his coronation or to the day the Ark of the Covenant was returned to Jerusalem. To the ancient Jews Psalm 118 it was likely a psalm sung during entrance to the temple, at the Passover. It celebrated God’s deliverance of the Jews from Egypt. It was also a psalm prophesying the future entrance of the Messiah into Jerusalem.
For Christians Psalm 118 has been related to Jesus’s triumphal entry on Palm Sunday, to the Resurrection, and to the Second Coming. Both Charles Spurgeon and Albert Barnes would have us apply the verse to every Sunday – and who can deny the appropriateness of the thought, as we should surely enter our churches with joy in our heart on every Sunday.
There are other more personal applications of the verse for me.
Firstly, I will always refer this verse to the day on which I got saved. Psalm 118 also says “I shall not die, but live, and declare the works of the Lord.” (Psalm 118:17), and I have known since that day that I shall not die but have everlasting life. I know, too, that it is laid upon me to declare the works of the Lord – and here I am doing it.
Secondly, I use the verse often, to remind myself of the goodness of God. As anyone who reads my daily devotion regularly will know there are times when I allow myself to become downcast. I have no right to let that happen.
Of every day it may be said, “This is the day which the Lord hath made”. There are no bad days – it is only a matter of my reaction. 
To me this is part of the business of being joyful. You can find the instruction to be joyful, in one way or another, in many scriptures. One of my favorites is Romans 12:12, part of a wonderfully brief prescription for the Christian life:

Be kindly affectioned one to another with brotherly love; in honour preferring one another; Not slothful in business; fervent in spirit; serving the Lord; Rejoicing in hope; patient in tribulation; continuing instant in prayer;  Distributing to the necessity of saints; given to hospitality. Bless them which persecute you: bless, and curse not.Rejoice with them that do rejoice, and weep with them that weep. (Romans 12:10-15).

Mostly, it’s about keeping a sense of proportion. For sure, there are things that are really unpleasant in life – but most of the things I fret about are not so important when considered as part of a whole life. Even the truly horrible parts of life have little or no eternal significance. But still, from time to time, I have to remind myself – “This is the day …”.