when can i get pregnant

the last two months i got my period both times on the 17th of september and october. i got my period this month on the 13th of november the exact day i was due my period. i had unprotected sex on the 17th of november as my period only lasted 4days. i cant remember wether i had sex on the 18th but i think i did. i had unprotected twice on the 23rd of this month , once on the 24th and once on the 25th all unprotected . i was wondering when should i be most fertile to get pregnant