The mishnah in Nedarim 8:3 gives the following general rule:
This is the rule: Anything that has a set time and he says, "until it reaches that time," he is forbidden until that time. If he says, "until it will be," he is forbidden until it passes. And anything that does not have a set time, whether he says "until it will be" or whether he says "until it reaches," he is only forbidden until it arrives.
זֶה הַכְּלָל, כֹּל שֶׁזְּמַנּוֹ קָבוּעַ וְאָמַר עַד שֶׁיַּגִּיעַ, אָסוּר עַד שֶׁיַּגִּיּעַ. אָמַר עַד שֶׁיְּהֵא, אָסוּר עַד שֶׁיֵּצֵא. וְכֹל שֶׁאֵין זְמַנּוֹ קָבוּעַ, בֵּין אָמַר עַד שֶׁיְּהֵא, בֵּין אָמַר עַד שֶׁיַּגִּיעַ, אֵינוֹ אָסוּר אֶלָּא עַד שֶׁיַּגִּיעַ:
Shavuos doesn't have a fixed date, both because "in the times of Kiddush HaChodesh it could come out as early as 5 Sivan and as late as 7 Sivan," and because depending on factors such as crossing the date line, one person's Shavuos can be on a different day as someone else's (don't have a source, it's just what my rabbi has said).
How does this work in the context of a neder? Is Shavuos considered a "set time" for this purpose?
