The word "police" is rather special: It has no singular noun form. Something like that police over there is securing the scene would be incorrect. One would always construct sentences in the plural form like so:
The police are out in force today.
Anything done by the police will reflect on them.
Other words that take no singular form would include pants, trousers, scissors, and clothes.
Confusion arises because "police" is also used as an adjective. Consider these sentences:
A police department is housed in that building.
The police chief was highly visible at the town meeting.
In these two sentences, we are not speaking of "a police". You could easily remove the word from both sentences and they would make sense semantically and grammatically. Instead, the word describes the department or chief. It gives us context.
"Police" also has a verb form. You may encounter it like this:
The Boy Scout troop must police the area before they leave to remove any trash.
The verb means "to investigate, to search, to clean up". This certainly does fit in with a subset of the duties of a police department.
"Series" can be singular or plural depending on context. Both of your sentences are therefore correct but different.
I like to watch TV series.
uses series as a plural and means you like to watch a number of different TV shows.
I like to watch a TV series.
uses series as singular and means you like to watch one TV show.
Best Answer
Since this is a weekly event, you have the option of either saying:
All singular:
or all plural:
You will find both usages are common in spoken and written English. In both business writing and conversation, it is very common to drop the plural and use all singular.
Here are some examples:
And some nice counterexamples:
That is a special event, not a regular occurrence.
Notice this happens on select(Definition) Tuesdays and Thursdays, so this is not an every week event -- use plural.
This example uses both -- Plural in the headline, then singular in the text. Singular is used because "Tuesday and Thursday" modify "mornings", which is the noun.
Notice the parallelism. Singular and plural remain constant in the same list -- they are not mixed in the same phrase.
Wait, what about the punctuation?
Some of the examples used dashes ("Tuesday–Thursday"), some used commas ("Monday, Wednesday and Friday") and some used slashes ("Tuesday/Thursday"). Does this matter?
Some readers will consider it the most correct to use commas, because it's a conventional list (Alice, Bob, Charlie, and Delia) but you'll often see people using dashes or slashes to separate days of the week, as well as other technical items (valid Tuesday/Thursday, with 1–4 gallons of water, using versions 1.0/2.0/3.5 of the policy.)
TLDR
Stick to commas and singular and you're fine.