"Just a quick reminder that the party is this week" works well as an informal wording for a party reminder. Other possible wordings include "Don't forget that the party is tomorrow at 7:00 p.m." or, "We're looking forward to seeing you at the party next week."
Including the specific date and time of the party as well as other pertinent details in the reminder card or message can be helpful for guests. For example, use, "Just a reminder that the company holiday party is this Friday at 6:30 p.m." instead of just, "See you at the party!"
Zazzle.com sells customizable party reminder cards as well as save-the-date cards and party invitations.