Someone related this question at the s'udas Purim:
Haman's decree was promulgated on the 13th of Nisan and would not take effect until the 13th of Adar following. Yet, Mord'chay urged Ester to seek redress from Achashverosh immediately. This would not be surprising, except that she had told him she had not been called to Achashverosh in a month, so was risking her life: why not wait until she would be called (which would surely happen with time to spare before Adar)?
Malbim (to 4:13-14) writes (translation taken from The Malbim Esther, by R. Jonathan Taub (Feldheim, 1998)):
Mordechai informed Esther of several
fundamental principles: ...
For any tragedy that befalls the Jews,
there is a specific time and day ready
for their deliverance by certain
prepared means. If these means are not
actualized, then the Omnipresent will
prepare other means to save them.
Since Esther was granted her position
as queen only in order to help the
Jews, and since the date for their
salvation had already been determined,
if she did not act immediately she
might miss the appointed time and the
Almighty would use other agents to
effect their deliverance.
The g'mara (12:2, with Rashi) notes that, had Achashverosh not sent out the decree that men rule their homes, the gentiles would have killed out the Jews long before Adar in their zeal to comply with the law to do so. Only because they questioned their king (because of the earlier law) did they forbear. So perhaps Mord'chay was worried that the gentiles would not wait for Adar.
The Y'rushalmi (B'rachos 1:1) says "כך היא גאולתן של ישראל בתחילה קימאה קימאה", that redemption starts little by little (and the Jews' did at that time, what with the Bigsan story and all), so perhaps Mord'chay saw a need to start the process going although it was yet early.