Are there any general reference tables on duolingo? Like the days of week, the months, numbers, etc
Or are we supposed to look them up on wikipedia or other sources?
In the vocabulary tab on the right hand side is a list of the vocabulary by skills. If you click on one of the skills like "Animals" you get a list of all the animal words you learned and if you learn the skill "Dates and Time" there will be a list of the words for the months and days of the week. But besides that there is almost no reference material like this on duolingo as far as I know.
They add some reference once in a while, but I believe this is not of high priority because you can easily find tables and explanations elsewhere. Good (and free!) exercises enveloped into an attractive and gamelike environment are much harder to find.