<xen:if is="!{$visitor.customFields.disableSnowfall}">
[Your JS here]
</xen:if>
<xen:if is="!{$visitor.customFields.disableSnowfall}">
[Your JS here]
</xen:if>
var now = new Date();
if (now >= new Date(now.getFullYear(), 11, 20) && now <= new Date(now.getFullYear() + 1, 0, 20)) {
// code
}