While some people have already received emails directly advising them of the promotion, this is not one that is dependent on receiving the email and everyone in an eligible country can take part.
Speaking of which, Google has now confirmed that this promotion is open to Play Store users in the US, the UK and Japan. That's currently the full extent to the list so you will need to be based in one of these locations to redeem the promotion. Or at least, have an active Google account that's registered in one of the locations.
If you received the email, then redeeming the $1 off promo is as simple as clicking the link that's included in the email.
For everyone else, you can take part by heading to the Google Play Store on the web, and when logged in to an account clicking on the "Redeem" option in the menu. From here you just need to enter the code and it will give you the option to apply the discount to your account for use at a later time.
Mobile users can add the code in much the same way by opening the app, clicking on the "Payments methods" tab and then on the "Redeem code" option. Again, just enter the code and confirm you want to add the promotion to your account.
Interested parties have up until February 28, 2019 to apply the code to their account and then will have up until March 1, 2019 to actually use it on a purchase.
Besides the restriction that the code only works in some locations, the only other major caveats are the type of content you can buy and the overall price.
For example, the code is only redeemable against the purchase of an app – it won't work on movies or anything else that's available to buy/rent from the Google Play Store.
As for the cost restriction, the app does need to be on sale for a minimum of $3 at the time of purchase for the code to take effect. So this is more of a discount on a paid app, then the opportunity to bag a $1 app for free.
The code itself is "VALENTINE2019" without the quotation marks for users in the US. Those in the UK have the opportunity to save £1 against the cost of a £3 or more app by using "VALENTINE2019GB" instead. For those based in Japan, the same discount and overall cost rules apply, although the code is "201902."
Coinciding with the promotion, Google is also currently running a "Love is in the air" themed app section on the Play Store. Here you will find app suggestions that you might be interested in for you and your valentine to both share, as well as some for those of us flying solo this year. You can check out the full selection of the love-themed and highlighted apps here.