When both husband and wife agree, mikveh immersion may be pushed off by a night or two. So you may delay your immersion as long as your husband agrees.
Do note, however, that the seder usually begins well after nightfall. With a little help, you may well be able to immerse without causing any delay. Telling your mother-in-law (or another female family member) about your immersion for this purpose is permissible.
Chag kasher vesameach!