How many ACUs do I have? Posted from sandbox

The Intro

What is an ACU? This challenge is based on the concept of money and since this is an international community, we all use different currencies, so in keeping with our spirit of inclusiveness, I have invented a new currency called Arbitrary Currency Units (ACUs) specifically for this challenge. In reality, the challenge is the same whether we are using Dollars, Pounds, Euros, Bitcoins, Lao Kip or Baked Beans.

The Idea

It's New Year. We all have ideas about how we can improve things in the coming year. Mine is to save some money but randomly putting money aside quickly becomes uninteresting so I am following up on something I read on a news site last year. Each week, I put the amount of ACUs equal to the week number into a tin and after 52 weeks I will have saved ACU 1378. That's enough for a nice golfing holiday somewhere in 2021!

The Problem

I need an incentive to keep me going with this so I need to know how much I have or will have in the tin on any given date during 2020. For example, if I enter a date before today I want to know how much was in the tin on that date. For a date after today I want to know how much I will have if I stay with the plan.

The Challenge

Write a program or function which will, when given any date in 2020 (apart from 30-31 Dec - see "The Details"), output the number of ACUs that are in my tin.

The Details


Each week in 2020 consists of exactly 7 days.
The first week of the year starts on 1 January 2020.
We are considering only weeks that are completely within 2020. The last week ends on 2020-12-29 so there are exactly 52 weeks and 30/31 December are not included.
Each week I will put a number of ACUs equal to the week number into my tin on the first day of the week, that is on the 1st January I will put 1 ACU into the tin, on the 8th January I will put 2 ACUs, etc. I will at no point take any ACUs out of the tin.


The Rules


Program or function - your choice.
Input in whatever convenient format but it must be a date. I have a feeling that accepting a day-of-year input may trivialise this challenge but I am open to other opinions on this.
Output however you like but it must be a number of ACUs or a string representation of same.
We are only talking about complete weeks in 2020 so you can assume that the input will not include 30 or 31 December. It's up to you how you deal with it if it does.
Don't forget that in 2020 February has 29 days.
Usual loopholes barred and this is code golf so the shortest program/function in each language will be the winner. All languages welcome and I will not be accepting an answer as per current consensus. Upvotes will decide who wins.


A small Python script that generates the expected output for each week is available here Try it online!. The output can also be printed to stick on the side of your tin if you decide to do the same :)
APL (Dyalog Extended), 20 19 bytesSBCS



Anonymous tacit prefix function taking [year,month,day] as argument.

2!∘⌊7÷⍨43816-⍨⌂days


Try it online!

⌂days calculate the number of days since 1899-12-31

43816-⍨ subtract 43816; days since 2019 12 18, i.e 14 days weeks extra

7÷⍨ divide by 7; weeks since 2019 12 18, i.e two weeks extra

⌊ floor; whole weeks since 2019 12 18, i.e two weeks extra

∘

2! count the number of ways to choose 2 out of that many

In other words, if \$d(x)\$ is the number of days since 1899-12-31, then the answer is

$${\Big\lfloor{d(x)-43816\over7}\Big\rfloor\choose2}=
{\Big\lfloor{d(x)-43823\over7}\Big\rfloor-1\choose2}=
\sum_{n=1}^{\big\lfloor\frac{d(x)-43823}7\big\rfloor}n$$