Sentinel's Card

A Sentinel's Card is a pledge gift given out by (guard) NPCs during the Love is in the Air seasonal event. These are combined into packages which are in turn hierarchically combined with other pledge packages for buffs and rewards.

All of the Love is in the Air pledge gifts are non-functional as regards their name and picture. You cannot read a Sentinel's Card.

Source
Sentinel's Card is one of two pledge items (the other is Pledge of Loyalty: Darnassus) that can be found inside a Pledge of Adoration: Darnassus or a Pledge of Friendship: Darnassus, given by Darnassus Sentinels as part of the Love is in the Air event. There is always one and only one of these two pledge items in each of these containers. (The Pledge of Friendship contains only the pledge item; the Pledge of Adoration contains functional gifts as well.) The odds of getting either pledge item appear to be about even, but in the short run you are likely to get a stack of one while trying for the other.

You will need one free inventory slot or a current stack of less than 20 Sentinel's Cards in order to remove a Sentinel's Card from its container. To remove the Sentinel's Card from its container, right click the Pledge of Adoration or Pledge of Friendship to open it.

Ingredient
If you have five or more Sentinel's Cards in your inventory, right clicking on any of them will consume five and create a Stack of Cards.

Inventory space
The Pledge of Adoration: Darnassus and Pledge of Friendship: Darnassus container items do not stack. Opening your Pledges of Friendship: Darnassus should save inventory space, since both pledge items stack 20 high. It is generally useful to NOT autoloot a Pledge of Adoration container - the Love is in the Air event items quickly consume any available inventory space. If you have autoloot turned on, you can hold down the shift key while right clicking the Pledge of Adoration to not autoloot it. You can then remove only the items that you need at the moment.

Be careful when combining these into packages, the Sentinel's Card pledge item stacks 20 high, but the Stack of Cards package item does not stack at all.

Darnassus Sentinels
There are only female s, no males, so gender identity issues not withstanding, your (male or female) character has to use a Cologne Bottle to get the "give a " dialog, if you want to gather these pledge items.

All of the Darnassus Sentinels are Night Elves. For the Love is in the Air gift items given by non-guard NPCs, the race of the NPC, not the location, determines which city's gift item you get. Since all of a city's guard NPCs are the same race, the distinction does not come into play for the pledge items.

Transferability
This is a conjured item. It cannot be sold at the Auction House, mailed nor put in a guild bank tab, but it can be given, sold, or traded with other player characters using the trade window. Unfortunately, that means you cannot pass these directly to an alt on the same account.

Duration
This item is primarily useful during Love is in the Air and the seven day duration of the item is longer than the event, so is not a handicap. It does prevent you from saving these for the following year.