an act of God	an event such as flood, lightning, earthquake etc. that is produced by natural forces
come rain or shine	used for saying that something always happens or smn. always does sth despite bad weather/conditions
down on your luck	having a long period of difficulty finding work and earning money
draw the short straw	be chosen to do sth unpleasant
have green fingers	be good at growing plants
let nature take its course	allow sth to develop without trying to influence it
no rhyme or reason	used for emphasising that there is no reason or explanation for sth
out of the blue	happening in a way that is sudden and unexpected, and doesn't seem connected with anything happened
the luck of the draw	used for saying people have limited control over what life brings them
touch wood	used when you have said that you have had good luck in order to prevent bad luck from happening to u