From Longman Dictionary of Contemporary Englishstrike while the iron is hotstrike while the iron is hotCHANCE/OPPORTUNITYto do something immediately rather than waiting until a later time when you are less likely to succeed → strike
Examples from the Corpus
strike while the iron is hot• Don't wait until tomorrow before you tell him, strike while the iron is hot!• So, it should strike while the iron is hot and go to the country as soon as possible. 