I have column ponedjeljak as char(1), it was declared in java:

private char ponedjeljak='0';

public char getPonedjeljak() {
return ponedjeljak;
}

public void setPonedjeljak(char ponedjeljak) {
this.ponedjeljak=ponedjeljak;
}

After calling in code setPonedjeljak('1') I will get in database value 't' instead of '1'. Problem was because I had method:

public boolean isPonedjeljak() {
return '1'==getPonedjeljak();
}

After I renamed method isPonedjeljak() to isOnPonedjeljak() insert work. Is this bug?

yes, it is a bug in your JavaBean

:-) Or it is "done by specification" :-))))