To "make hay while the sun shines" means to make the most of a favourable situation while it lasts.
A similar situation would be that in time of peace, be prepared for war.
All that might initially seem to be a contraction or a paradox, but it is essentially about being prepared for rough times during good times.
In the 1st reading, St. Paul talks about the Day of the Lord coming like a thief in the night.
It is like when people are saying "How quiet and peaceful it is" that the worst suddenly happens.
That reminds us that calm can turn to chaos just like that and so we need to be prepared.
When it is calm and peaceful, we must take the opportunity to pray and read the Scriptures and be in communion with the Lord while we have the time to do so.
Because when the storm suddenly strikes and all is chaos, when we are too distressed to even pray, then it will be the Lord who will come to our aid and give us strength.
Let us be alert in prayer when it is peaceful, and the Lord will keep us from breaking into pieces during chaos.