These are articles are good introduction to panchang and Muhurta astrology respectively -
These are good introductory articles and must read for astrology beginners, especially students from the Muhurta branch of Vedic astrology.
I reached these websites while I was trying to find out the authentic websites that gives Muhurta for Laxmi Pujan this Deepawali (2008). I didn't find much agreement and I think this is definitely a topic of detailed article. Though there is not much time before Diwali so you will have to wait till next Diwali ;-)
Anyways, it is important to understant that the two most important parts of Diwali Laxmi Pujan muhurta calculation are sunrise time (for calculating pradosh period) and ascendant (lagna). Both of these varies from location to location. So the muhurta for Delhi will not be same as muhurta for Agra. It is important you consult local panchang, local news paper or local astrologer so that you know exact muhurta for your location.