'What day is it?' has become something of a symbolic question for the overly confused, and for good reason. You would soon find yourself in a pickle if you couldn't remember, or find out, what day of the week, month of the year, or even time of the day it is. So, before you start testing that body clock with a night on the Piwa (Beer [pl]), it's not a bad idea to familiarise yourself with some expressions of time, and some of the grammar changes that will let you make plans and recall activities at specific times in the past or future.
Days of the Week
The Polish word for 'week' is tydzień, and it comprises - surprisingly just as in English - of six days: Poniedziałek (Monday), Wtorek (Tuesday), Środa (Wednesday), Czwartek (Thursday), Piątek (Friday), Sobota (Saturday), and Niedziela (Sunday).
The Polish word for 'day' is dzień, and its plural 'days' is dni; the plural for 'week' is tygodnie.
The words for the days of the week also have feminine renditions in Polish, and these are found by simply removing the -ek endings on the days that have them, the others do not change.
Saying 'On' a Day
To say something happened on a particular day, we need to use the preposition w (which has a literal translation something more like 'in', but the Polish word for 'on' [na] is used to mean physically on something; so try to imagine a day in Polish like a container, that contains your activities - they are literally 'in' them). Whenever we use this preposition we also need to alter the ending of the associated noun - which here are the days of the week - to the accusative case endings; they are as follows:
W poniedziałek (on Monday), we wtorek (on Tuesday), w środę (on Wednesday), w czwartek (on Thursday), w piątek (on friday), w sobotę (on saturday), and w niedzielę (on Sunday).
Actually the accusative endings for the days are quite easy, because there aren't that many changes to be made; just the -ę ending for the feminine nouns for Wednesday, Saturday and Sunday.
Saying Particular Times of the Day
From morning to midday, evening to night, here's some useful vocabulary that will let you further refine your sentences and speaking skills:
Dziś or Dziśiaj means 'today', jutro means ‘tomorrow’ (do jutra means during tomorrow, and you will probably need this preposition for most common sentences), and wczoraj means 'yesterday'.
Rano is 'morning', wiec is 'evening' (hence dobry wieczor 'good evening'), and noc is 'night' (hence dobranoc 'goodnight'), while południe is noon and - this one should be easy to remember - popołudnie is 'afternoon'.
Again, if you want to say something is happening at a particular time using the vocabulary above then you will need to use the correct preposition and case ending for that word. So, wieczorem means 'in the evening', po południe is the gunslinger's favourite, 'at noon', and o północy means 'at midnight'.
This is such common vocabulary that the ending changes, which on paper may look relatively unpredictable and sporadic, will soon become second nature; it will see normal to add them when you are describing what you have done, or what you will and when.