exactly when did Israel formally declare war on Syria,
"Upon independence, Israel was invaded by the armies of six Arab nations: Egypt, Syria, Transjordan (later Jordan), Lebanon, Iraq and Saudi Arabia. In addition, local Arab Palestinian forces also fought the Jewish Israelis. Israel held off the Arab forces and established itself as an independent nation. From the start of this war, Syria and Israel have been engaged in one continuous legal state of war. While technically at war this whole time, in reality, their conflict has been punctuated by several major (though short), wars and numerous cross-border attacks and air battles."

any member can invade any country if the UN has stated that something that country is doing is illegal
No-one has been invaded, they just stopped a serious violation.